What does metallic-smelling stool indicate? 3 causes and how to treat them

Stool with a metallic smell or a smell like iron or rust can cause concern for many people, especially when it appears suddenly. In reality, changes in stool odor can be related to food, gut microbiota, medications being used, or certain digestive issues. Not every instance of stool with a metallic smell is a sign of a serious illness, but if the unusual odor persists or is accompanied by changes in stool color and consistency, you should not ignore it.

This article helps you quickly understand why stool has a foul or metallic smell, common causes, and which signs require attention. Tiptory will focus on practical, easy-to-apply information so you know when you can monitor at home and when you should consult a healthcare professional.

What to note about metallic-smelling stool?

Metallic stool odor can be linked to many causes

• Metallic-smelling stool does not necessarily mean the body has excess iron. Stool odor is influenced by food, gut microbiota, and substances in the digestive tract. If you are taking iron supplements, your stool may turn dark or black, so it is important to evaluate both the color and consistency of the stool rather than just relying on the odor.

• When you have metallic-smelling stool accompanied by a small amount of bright red blood on toilet paper or on the outside of the stool, the cause may be in the anal area, such as hemorrhoids or an anal fissure. Bright red blood typically comes from the lower part of the digestive tract, but you should not self-diagnose based solely on the color of the blood.

How to recognize signs that require a doctor's visit

1. Observe stool color and blood amount: If only a small amount of bright red blood appears once, especially after constipation or straining during bowel movements, you can monitor it but should still consult a healthcare professional if the condition recurs.

2. Pay attention to dark red or black stool: Black, very dark, and tar-like stool can be a sign that blood has been digested from the upper part of the digestive tract. However, iron pills and some foods can also turn stool black, so the specific cause needs to be considered.

3. Do not ignore accompanying symptoms: Foul-smelling stool or unusual odor needs more attention if it is accompanied by abdominal pain, prolonged diarrhea, fatigue, unintentional weight loss, or significant changes in bowel habits. These signs may require a doctor's evaluation to find the cause.

When to seek early medical care?

• Bowel movements with black or dark red stool, especially when not explainable by iron supplements or food.

• Repeated or increasing amounts of blood in the stool.

• Heavy, unstoppable bleeding or large blood clots.

• Metallic-smelling stool accompanied by significant abdominal pain, dizziness, severe weakness, or other unusual symptoms.

If there is heavy or continuous bleeding, emergency medical attention is needed instead of just monitoring at home.

Important points to remember

• The metallic smell of stool is not sufficient to determine the cause.

• Assess simultaneously stool color, presence of blood, stool consistency, and accompanying symptoms.

• Especially, tar-like black stool or dark red stool needs attention as it may be related to gastrointestinal bleeding.
  

Part 1: Common causes of metallic-smelling stool

Note 1: Metallic smell in stool: When should you be concerned?

Metallic smell does not equate to iron overload

• Metallic-smelling stool may immediately make you think of iron or blood, but relying solely on the smell cannot determine the cause. Stool odor is also influenced by food, gut microbiota, and many other digestive factors.

• If you are taking iron medication or supplements, your stool may become dark or black. Therefore, when experiencing metallic-smelling stool, observe both the stool color and accompanying symptoms instead of concluding that your body has excess iron.

Bright red blood in stool usually originates from the lower digestive tract

• If you see bright red blood on toilet paper, on the outside of the stool, or in the toilet bowl, the cause may be related to the anal area, such as hemorrhoids or an anal fissure. These are common causes, but you should not self-diagnose if the condition recurs.

• If metallic-smelling stool is accompanied by bright red blood, monitor the amount of blood and other symptoms. Severe abdominal pain, dizziness, unusual fatigue, or recurrent bleeding are reasons to be evaluated by a healthcare professional.

Black or dark red stool needs more attention

• Black, very dark, or tar-like stool can be a sign of digested blood from bleeding in the upper part of the digestive tract, such as the stomach or duodenum. This condition requires early medical evaluation.

• However, black stool is not always due to bleeding. Iron supplements and some dark-colored foods can also turn stool black. Therefore, the medications being used, diet, and accompanying symptoms need to be considered.

If no blood is seen, other factors still need to be considered

• If metallic-smelling stool is present but no blood is seen, the unusual odor may be related to changes in diet or the activity of the gut microbiota. Do not rely solely on the smell to identify an illness.

• Note recent changes such as new foods eaten, medications or supplements being used, constipation, diarrhea, and changes in stool color. This information helps the doctor more easily assess the cause.

When to see a doctor early?

• Persistent foul-smelling or metallic-smelling stool accompanied by blood in the stool.

• Stool turning black or dark red without being explainable by iron supplements or food.

• Heavy bleeding, continuous bleeding, or large blood clots.

• Accompanied by abdominal pain, dizziness, severe weakness, or feeling distinctly unwell.

• If the amount of blood is large or bleeding is continuous, seek emergency medical care instead of continuing to monitor at home.

Note 2: Iron-rich foods can change stool odor

Diet can affect stool odor

• If stool has a metallic smell but is still a normal brown color, without blood, and no other unusual symptoms, one possibility is that the odor change is due to diet. Stool odor is influenced by the digestive process and compounds produced in the intestines, so some foods can cause a temporary unusual stool odor.

• Especially if you have recently eaten a lot of iron-rich foods, consider this a possible contributing factor before becoming overly concerned about metallic-smelling stool.

Check recently eaten foods

• In the days leading up to noticing metallic-smelling stool, review your diet. Some iron-rich foods include:

  • Red meats like beef, lamb, and veal.

  • Organ meats like liver, kidneys, and processed organ products.

  • Chicken, fish, and eggs.

  • Nuts and dried fruits.

  • Oats, tofu, and whole-grain products.

• The iron content in food does not mean that eating these foods will definitely cause foul-smelling or metallic-smelling stool. This is just one factor that can affect stool odor in some people.

Monitor changes after adjusting your diet

1. Note any iron-rich foods you have eaten in the past few days.

2. Maintain a balanced diet and avoid changing too many types of food at once, to make it easier to identify which factor might be related to the change in stool odor.

3. Monitor the color, shape, and odor of your stool in the following days.

4. If metallic-smelling stool gradually decreases after your diet returns to normal and no blood or unusual symptoms appear, there is usually no need to worry excessively.

Do not attribute every metallic smell to food

• If metallic-smelling stool persists despite dietary changes, or if tar-like black stool, bright red or dark red blood in the stool, abdominal pain, dizziness, or unusual fatigue appear, you should be evaluated by a healthcare professional.

• Especially, foul-smelling stool accompanied by significant changes in color or blood should not be solely explained by eating iron-rich foods. Stool odor is like a "secondary signal"; both stool color and accompanying symptoms need to be considered to understand the issue correctly.

Note 3: Medications and supplements can change stool odor

Medications or supplements can affect stool odor

• If metallic-smelling stool appears after you start a new medication or supplement, this could be a related factor. Some medications can change the gut microbiota or stool composition, thereby making the stool odor different from normal.

• Specifically, iron supplements can turn stool dark green or black and sometimes come with other digestive changes. Therefore, if you experience metallic-smelling stool after starting iron supplements, check the product's ingredients and when the symptoms appeared.

Antibiotics can change stool odor

• Antibiotics alter the bacterial flora in the intestines. This change can affect fermentation processes and produce different odor-causing substances in the stool.

• Some people notice foul-smelling or unusual-smelling stool while taking antibiotics. Metronidazole can also cause taste changes and some digestive side effects, but it should not be assumed to be the cause of all cases of metallic-smelling stool.

Check current medications and supplements

1. Review newly started medications: Note the name of the medication, when you started taking it, and when the metallic stool odor appeared.

2. Check supplement ingredients: Pay special attention to iron in multivitamins, mineral supplements, or individual supplements.

3. Do not stop medication on your own: If you suspect medication is causing changes in stool odor, talk to your doctor or pharmacist before changing the dose or stopping the medication, especially for prescription drugs.

4. Monitor other symptoms: Pay attention to diarrhea, abdominal pain, fever, bloody stool, or unusual black stool. These signs are more important than the stool odor itself when evaluating digestive health.

Do not confuse stool odor with signs of bleeding

• A metallic odor alone is not sufficient to determine if there is blood in the stool. If bright red blood, dark red stool, or tar-like black stool appears, the cause needs to be evaluated rather than assuming it is just a side effect of medication.

• If metallic-smelling stool is accompanied by dizziness, severe weakness, significant abdominal pain, or gastrointestinal bleeding, seek early medical care.

Expert note on medication use

• If the odor appears immediately after starting medication or supplements and there are no warning signs, note the timing and discuss it with a healthcare professional if the condition persists.

• Do not self-medicate to "eliminate stool odor." The most important thing is to identify the cause of the change and rule out any abnormal signs of the digestive tract.

Part 2: When should you see a doctor for unusual-smelling stool?

Note 1: Metallic smell and blood in stool

Metallic odor alone is usually not an emergency sign

• If metallic-smelling stool is present but without blood, abdominal pain, or other unusual symptoms, you can monitor changes in your diet, medications, and supplements.

• However, the severity should not be judged solely by the odor. Metallic-smelling stool needs to be considered along with stool color, amount of blood if any, and accompanying symptoms.

Bright red blood may be related to the anal area

• Bright red blood on toilet paper, on the outside of the stool, or in the toilet bowl usually originates from the lower part of the digestive tract. Hemorrhoids and anal fissures are common causes, especially with constipation or straining during bowel movements.

• Although a small amount of blood appearing only once is usually not an emergency, rectal bleeding should still be evaluated if it recurs or causes you concern. Do not assume all cases of bright red blood are hemorrhoids.

Some accompanying symptoms need attention

• If foul-smelling or metallic-smelling stool is accompanied by abdominal pain, dizziness, lightheadedness, severe fatigue, or shortness of breath, you should seek early medical care. These could be accompanying symptoms of gastrointestinal bleeding or another problem that needs to be checked.

• Specifically, tar-like black stool or dark red stool can be related to gastrointestinal hemorrhage and requires timely medical evaluation.

Do not self-diagnose the cause as hemorrhoids

• In addition to hemorrhoids and anal fissures, blood in the stool can also be related to colorectal polyps, inflammatory bowel disease, diverticular disease, infections, or other digestive tract conditions. Therefore, if the condition recurs, the doctor may need to take a medical history, perform an examination, and conduct appropriate tests to find the cause.

When to see a doctor immediately?

1. Stool turns black or dark red, especially if not explainable by iron supplements or food.

2. Heavy or continuous bleeding, toilet water turning red, or large blood clots appearing. This is a situation that requires emergency medical care.

3. Metallic stool odor accompanied by severe abdominal pain, dizziness, fainting, or severe weakness also needs rapid evaluation as it may be accompanied by significant gastrointestinal bleeding.

• Expert note: unusual stool odor is not a diagnostic tool. What is more important is to recognize changes in stool color, blood in the stool, and systemic symptoms to decide whether a doctor's visit is needed.

Note 2: Persistent metallic-smelling stool requires examination

Black, tar-like stool is a sign to watch out for

• If metallic-smelling stool is accompanied by black, sticky, or tar-like stool, do not only think about food or supplements. Black stool can be a sign of gastrointestinal bleeding, especially when blood is bleeding in the upper part of the digestive tract.

• However, iron supplements can also turn stool black. Therefore, the medications being used and other symptoms need to be considered rather than self-diagnosing the cause.

Persistent stool odor should be evaluated

• If metallic-smelling stool persists even after dietary or medication changes as advised by a doctor, you should see a doctor to find the cause instead of continuing to monitor yourself.

• Foul-smelling or very strong-smelling stool can occur in many different digestive conditions. Stool odor alone cannot determine what illness you have; the doctor will also rely on stool color, stool consistency, accompanying symptoms, and medication history to make an assessment.

Some medical conditions may be related to stool changes

• Conditions such as malabsorption, Crohn's disease, celiac disease, gastrointestinal infections, or pancreatic disease can cause changes in stool consistency and odor, especially when accompanied by prolonged diarrhea, abdominal pain, or weight loss.

• It should not be assumed that metallic-smelling stool indicates cancer. Cancer is one of many possible causes of bleeding or changes in bowel habits, but examination and testing are necessary to determine the cause.

Signs requiring early medical attention

1. Black or dark red stool, especially if not clearly related to iron supplements or food.

2. Metallic-smelling stool accompanied by abdominal pain, dizziness, lightheadedness, severe fatigue, or shortness of breath. These can be symptoms accompanying gastrointestinal bleeding and require prompt evaluation.

3. Heavy or continuous bleeding, with water in the toilet turning red or large blood clots appearing. This is a situation requiring emergency medical care.

4. Persistent unusual stool odor along with diarrhea, changes in bowel habits, abdominal pain, or unintentional weight loss. The doctor may order blood tests, stool tests, endoscopy, or imaging tests depending on the condition.

• Experts note: stool odor is only a signal, not a diagnostic tool. When passing metallic-smelling stool, prioritize checking the stool color, presence of blood, and accompanying symptoms. Specifically, tar-like black stool or bleeding accompanied by dizziness and abdominal pain requires prompt medical attention.

Part 3: How to fix foul-smelling stools

Note 1: Reduce high-sulfur foods if stool smells foul

Sulfur-rich foods can make stool smell strong

• If your stool has a foul odor, especially like rotten eggs, a diet high in sulfur-containing compounds may be a contributing factor. When gut bacteria break down these compounds, they can produce distinctive smelling gases.

• Some common sulfur-rich foods include cabbage, kale, potatoes, garlic, onions, and dried fruits. These are all nutritious foods and do not need to be completely eliminated from the diet.

Adjust portions instead of strict avoidance

1. Review your diet: If you've recently eaten a lot of sulfur-rich foods and notice foul-smelling stool, try reducing your intake for a short period.

2. Maintain a diverse diet: Do not eliminate many food groups simultaneously just because your stool odor changes. Vegetables, fruits, whole grains, and fiber-rich foods still play an important role in gut health.

3. Monitor changes: Observe stool odor after adjusting your diet. If the odor gradually decreases without other symptoms, diet may be one of the related factors.

4. Do not confuse rotten egg smell with metallic smell: Metallic-smelling stool and rotten egg smell do not necessarily have the same cause. Metallic-like odor needs to be considered along with stool color, medications being taken, and the possibility of blood in the stool.

Sulfur is still essential for the body

• Sulfur is an essential element involved in the structure of some amino acids and proteins. Therefore, the goal is not to cut back on sulfur as much as possible but to maintain a balanced, diverse diet.

• If metallic-smelling stool or foul-smelling stool persists despite dietary changes, or if persistent diarrhea, abdominal pain, unintentional weight loss, black stool, or blood occurs, you should consult a healthcare professional to find the cause instead of continuing to self-adjust your diet.

Note 2: Supplement fiber to aid digestion

Fiber helps maintain bowel function

• Fiber plays an important role in the gut microbiome, stool formation, and bowel movements. For people experiencing loose stools, constipation, or irregular bowel movements, adjusting fiber intake can help improve digestive function.

• Adults should generally aim for about 22–34 grams of fiber per day, depending on age and gender. Specific needs may vary between individuals.

Prioritize fiber from natural foods

• If your stool has a metallic odor or foul odor accompanied by irregular bowel movements, prioritize adding fiber from:

  • Green vegetables and root vegetables.

  • Whole fruits.

  • Beans and nuts.

  • Oats, brown rice, and whole grains.

• Increase fiber intake gradually rather than suddenly. The body needs time to adapt; increasing too quickly can cause bloating, flatulence, or excess gas.

Drink enough water when increasing fiber

1. Start by adding a serving of vegetables, fruits, or whole grains to your daily meals.

2. Increase fiber intake gradually over several days or weeks, depending on tolerance.

3. Drink enough water throughout the day so that fiber can retain water and help stool become soft and easier to pass.

4. Monitor bowel movements, especially if you previously had constipation, loose stools, or irregular bowel movements.

Consider psyllium fiber when needed

• Psyllium is a type of soluble fiber that can absorb water and form a gel in the intestines. It is often used to help regulate bowel movements and improve constipation or irregular stools.

• If using psyllium, follow the product instructions carefully and drink enough water. Individuals with difficulty swallowing, unexplained abdominal pain, or abnormal changes in bowel habits should consult a healthcare professional before use.

Fiber is not a treatment for abnormal stool odor

• Increasing fiber can support digestive health but cannot identify or treat the cause of metallic-smelling stool. If metallic-smelling stool persists, especially when accompanied by black stool, blood in stool, abdominal pain, persistent diarrhea, or unintentional weight loss, you should seek medical attention to find the cause.

• Experts recommend viewing fiber as part of a balanced diet, rather than increasing a lot of fiber just to try to eliminate foul-smelling stool.

Note 3: Drinking enough water aids better digestion

Adequate hydration supports the intestines

• Water is essential for many bodily functions, including digestion and maintaining stool softness. When the body is dehydrated, stool can become dry, hard, and difficult to pass.

• If you are experiencing metallic-smelling stool or foul-smelling stool, drinking enough water can support digestive health but should not be considered a direct treatment for the underlying cause of the odor.

Drink water regularly instead of waiting until you're thirsty

1. Drink water throughout the day: Divide your water intake into multiple portions instead of drinking a large amount at once.

2. Prioritize plain water: You can drink water during or after meals if your body tolerates it well. It's not necessary to adhere to a fixed number like 6–8 glasses per day, as water needs vary with age, activity level, weather, diet, and health status.

3. Increase water intake when needed: When the weather is hot, you're exercising a lot, or you're dehydrated due to vomiting or diarrhea, your body may need additional fluids.

4. Monitor for signs of dehydration: Excessive thirst, dry mouth, infrequent urination, or dark yellow urine may suggest your body needs more water.

You can add flavor to make it easier to drink

• If you find it difficult to maintain the habit of drinking plain water, you can add a few slices of lemon, orange, or other fresh fruits to add flavor.

• Unsweetened sparkling water can also be an option for some people, but if you are prone to bloating or belching, carbonated drinks may worsen these uncomfortable symptoms.

Combine water with fiber to aid bowel movements

• If you are increasing fiber to improve constipation or irregular bowel movements, adequate water intake is also crucial. Fiber needs water to help form soft and easier-to-pass stools.

• However, if metallic-smelling stool persists or foul-smelling stool is accompanied by blood, black stool, abdominal pain, prolonged diarrhea, or unintentional weight loss, drinking more water does not replace seeking medical examination and finding the cause.

Part 4: Why does stool smell strongly of rotten eggs?

Rotten egg smell is often associated with sulfur compounds

• If your stool has a foul odor, especially like rotten eggs, a common cause is sulfur-containing compounds produced during digestion. The gut microbiome can break down certain food components and create characteristic smelling gases.

• Some foods high in sulfur compounds include onions, cabbage, broccoli, cauliflower, carrots, and potatoes. Eating a lot of these foods can contribute to changes in stool odor in some people.

If no diarrhea, review your diet

• When stool smells like rotten eggs but still has a relatively normal color and shape, with no diarrhea, fever, or abdominal pain, you can review the foods you've eaten in the past few days.

• There's no need to completely eliminate sulfur-rich vegetables. These are still foods that provide important fiber, vitamins, and minerals. Instead, you can adjust your portions and monitor whether the stool odor improves.

Foul-smelling stool like rotten eggs accompanied by diarrhea needs attention

• If foul-smelling stool occurs along with diarrhea, abdominal pain, bloating, or nausea, you should not assume the cause is solely due to eating a lot of sulfur-containing foods.

• Giardiasis (Giardia infection) is an intestinal infection that can cause diarrhea, abdominal cramps, bloating, and abnormal stools. It is often associated with ingesting Giardia from contaminated water or food. Not all cases of rotten-egg-smelling stool with diarrhea are giardiasis, so testing is necessary if a doctor suspects it.

What should you do if your stool smells like rotten eggs?

1. Review foods: Note sulfur-rich foods eaten in recent days.

2. Monitor symptoms: Pay attention to diarrhea, abdominal pain, fever, nausea, and dehydration status.

3. Drink enough water: If you have diarrhea, prioritize fluid replenishment to reduce the risk of dehydration.

4. Seek medical attention if diarrhea is prolonged or severe: Your doctor may order a stool test to look for Giardia or other infectious causes.

• Experts note: metallic-smelling stool and rotten-egg-smelling stool are two different odor presentations and may have different causes. Stool odor is only a reference sign; stool color, consistency, and accompanying symptoms are more important information when assessing digestive health.

Part 5: Causes of ammonia-smelling stool

Ammonia odor may be related to the digestive process

• If your stool has an ammonia odor or smells like urine, the cause may be related to food composition and the process of gut bacteria breaking down protein. Stool odor can temporarily change when the diet changes.

• It should not be immediately concluded that ammonia-smelling stool is due to "gut bacterial imbalance." The gut microbiome is very complex, and stool odor alone is not sufficient to determine if there is an overgrowth of bacteria.

Dietary changes can alter stool odor

• If the odor appears for a short period after a dietary change, review the amount of meat, eggs, dairy, beans, and other protein-rich foods you are consuming.

• You can maintain a diverse diet, supplement with appropriate vegetables and fiber, and drink enough water. Do not arbitrarily completely eliminate protein or an important food group just because stool odor changes.

Persistent odor needs to be investigated

• If ammonia-smelling stool persists for many days or recurs frequently without obvious dietary changes, you should consult a healthcare professional.

• Specifically, if the unusual odor is accompanied by abdominal pain, prolonged diarrhea, unintentional weight loss, jaundice, edema, changes in urine output, or unusual fatigue, it needs to be checked to rule out gastrointestinal diseases and other health problems.

Do not confuse ammonia odor with signs of liver or kidney disease

• Liver or kidney disease can cause changes in metabolism and the composition of waste products in the body, but ammonia stool odor is not a sufficiently specific sign to diagnose liver or kidney disease.

• Therefore, if metallic-smelling stool, foul-smelling stool, or ammonia odor persists, evaluate the stool color, shape, and accompanying symptoms instead of relying solely on the odor.

What should you do first?

1. Note when the unusual odor started.

2. Review recent changes in diet, medications, and supplements.

3. Monitor stool color, shape, and frequency of bowel movements.

4. If the odor does not improve or unusual symptoms appear, seek medical attention for an appropriate evaluation of the cause.

• Experts note: changes in stool odor for a few days do not necessarily mean a serious medical condition. More concerning are persistent, recurrent odors or those accompanied by other significant changes in the digestive system.

References

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Content editor: Rowan Hudson Le.

Information reviewed and verified by expert: Michael Anderson.