A sudden metallic smell from your poop can make you pause and worry. If it reminds you of blood, it's understandable to wonder whether something is wrong.
Odor alone doesn't tell you much about bleeding. Food, supplements, medications, and changes in your sense of smell are more useful places to start.
What Makes Poop Smell?
Most stool odor comes from bacteria in your colon breaking down what your small intestine didn't absorb. What you smell are the resulting byproducts.
Sulfur compounds are among the main contributors. When researchers analyzed human stool with gas chromatography, the dominant odor-producing molecules were methyl sulfides, not the indole and skatole long assumed responsible.
Sulfur compounds can produce odors people describe in different ways. Metallic, chemical, like pennies, or like a swimming pool: these are subjective descriptions, and there is no established chemical signature for metallic stool odor. Urine odor can also change for ordinary reasons.
“Metallic” is a description, not a finding. No research has established that metallic stool odor points to one specific cause, so start with what changed recently and what else you're noticing.
Where to Start
Stool odor can change when different foods reach your colon, so think back through the last few days of eating.
A jump in protein. Unabsorbed protein is fermented in the lower colon, and protein fermentation produces sulfides, ammonia, and phenols. A steak-heavy weekend, new protein powder, or low-carb diet can change stool odor.
Sulfur-rich foods. Broccoli, cabbage, Brussels sprouts, garlic, onions, and eggs contain sulfur compounds that can contribute to stool odor. If the smell is closer to rotten eggs than metal, sulfur-smelling stool has its own set of causes.
Less fiber alongside more protein. Fermentable carbohydrate gives gut bacteria another source of fuel. Cutting fiber while increasing protein may change the mix of compounds produced during fermentation.
A food-related odor change should improve after the dietary trigger is gone. If it persists without an obvious dietary explanation, consider the other possibilities below.
Iron, Bismuth, and Anything Else You Take Daily
Anything you take regularly can change stool chemistry. Two common products are worth checking.
Iron. Ferrous sulfate commonly darkens stool, often to a greenish-black color. One study in people with iron deficiency anemia found that fecal volatile compounds changed after oral iron therapy, though researchers measured chemistry rather than perceived smell. Iron may change stool odor, but it has not been shown to produce a specifically metallic one.
Bismuth subsalicylate. Pepto-Bismol and similar products darken stool and tongue because bismuth reacts with sulfide in your gut to form bismuth sulfide, a black insoluble salt. The discoloration is temporary and generally harmless.
If iron was prescribed to you, a change in color or smell is a reason to talk with your prescriber, not to stop taking it.
This is also where tracking can help. Throne clips onto your toilet and automatically records bowel movement patterns without manual logging.
There Could Be an Issue in Your Nose
The change may be in how you perceive smell rather than in what you're producing.
Parosmia and phantosmia are smell disorders in which familiar odors are perceived differently or an odor is perceived without an obvious source. Both can follow viral upper respiratory infections, including COVID-19. Burnt is the smell most commonly reported in phantosmia, with rotten, musty, and metallic also reported.
Notice whether other familiar smells, like coffee, soap, or your own cooking, seem off too. If they do, the change may be in your sense of smell. It can also help to ask whether someone else notices anything unusual.
If your sense of smell is the only thing that seems off, an ear, nose, and throat specialist is a better place to start than a gastroenterologist.
Infections and Absorption Problems
When an odor change comes with changes in texture, frequency, or how you feel, those changes are more useful clues than the smell itself.
Sudden, foul, and watery. Gut infections can change the mix of volatile compounds in stool, and the patterns differ in Clostridioides difficile and Campylobacter compared with healthy stool. Fever, cramping, recent antibiotics, or recent travel can provide additional clues.
Pale, greasy, floating, hard to flush. That combination can indicate steatorrhea, fat that wasn't properly absorbed, which produces pale, bulky, malodorous stools. Causes include pancreatic disease, celiac disease, or problems affecting bile flow. Bloating, weight loss, and persistent diarrhea can occur alongside it.
If either pattern persists or comes with significant pain, fever, dehydration, or weight loss, contact a doctor.
What Digested Blood Actually Looks Like
If you're wondering whether the smell could mean bleeding, here's what to look for.
Blood that remains in the gut long enough can produce melena: black, tarry stool with a strong, characteristic odor. Transit time plays an important role, which is why bleeding further down the small bowel can also produce melena.
Even relatively small amounts of blood can cause melena. The amount varies with where and how quickly the bleeding occurs, so there isn't a useful cutoff to rely on.
Color and texture tell you more than smell. Black, sticky, or tar-like stool, or visible red blood in or on the stool, should be medically evaluated. Iron and bismuth can also darken stool without causing the characteristic tar-like consistency of melena.
Don't try to sort this out by smell. If you have black, tarry stool or visible blood, contact a doctor promptly. Seek emergency care if bleeding is accompanied by dizziness, fainting, a racing heart, shortness of breath, or cold, clammy skin.
