Antibiotics can change the color of your stool, including making it darker, and in most cases the shift is harmless. The explanation usually traces back to how these drugs alter gut bacteria that process bile pigments, or to chemical reactions between certain antibiotics and dietary iron. That said, dark or bloody stool during an antibiotic course can sometimes signal a genuine complication worth medical attention, so understanding the difference matters.
How Antibiotics Alter Stool Color Through Bile Pigments
Your stool gets its normal brown color from a pigment called urobilinogen, which is the end product of a chain of events that starts with bile. Your liver excretes bile into the intestine to help digest fats, and bacteria in your colon then break down the bile pigment bilirubin into urobilinogen. That urobilinogen is what gives stool its characteristic brownish hue. When antibiotics kill off large numbers of those gut bacteria, the whole process gets disrupted.
A study examining the effects of several oral antibiotics on bile pigment processing found that bacitracin, vancomycin, clindamycin, erythromycin, and ampicillin all caused a pronounced drop in urobilinogen output in the stool. Clindamycin additionally led to a marked increase in conjugated bilirubin reaching the feces, along with changes in the chemical breakdown pattern of bilirubin derivatives.1PubMed. Influence of antibiotics on the faecal excretion of bile pigments in healthy subjects In plain terms, instead of bilirubin being fully converted into brown-colored urobilinogen, partially processed or unconverted bilirubin ends up in the stool. Bilirubin itself is yellow-green to dark greenish, so depending on how much accumulates, you can see stool shift toward green, dark green, or a much darker shade than usual. This is a direct consequence of the antibiotic disrupting the normal bacterial ecosystem in the colon.
The color change can persist throughout the antibiotic course and sometimes for days or weeks after you finish, because gut bacteria need time to repopulate. It does not mean the antibiotic is harming you. It simply means the microbes that usually handle bile processing are temporarily knocked back.
The Cefdinir and Iron Reaction
One of the most dramatic and alarming stool color changes during antibiotic use involves cefdinir, a cephalosporin commonly prescribed for ear infections, sinusitis, and bronchitis, especially in young children. When cefdinir is taken alongside iron-containing products, including iron-fortified infant formulas or iron supplements, the drug or one of its breakdown products binds with ferric iron to form a non-absorbable complex. That complex turns stool a vivid reddish or maroon color that parents frequently mistake for blood.2PubMed. Nonbloody, red stools from coadministration of cefdinir and iron-supplemented infant formulas
This reaction has sent many worried families to the emergency department. A case series documented three infants who were brought in for evaluation of what their parents described as “bloody diarrhea.” In each case, the children had been started on oral cefdinir while also receiving iron-containing formula. Testing confirmed the stools were negative for blood, and the red color was entirely due to the drug-iron chemical reaction.3PubMed. Emergent “Bloody Diarrhea” Associated with the Use of Oral Cefdinir in Young Children: A Brief Report and Review of Literature The authors emphasized that a careful medication and dietary history can prevent unnecessary testing and spare families significant anxiety.
The reaction can go beyond stool color. A case report of a four-year-old girl described the child’s stool turning red during three separate courses of cefdinir mixed with an iron-fortified nutritional formula. Even the formula itself turned purple after cefdinir was mixed into it, a previously unreported observation that reinforced the chemical nature of the interaction.4PubMed Central. Cefdinir-induced red stool and purple discoloration of nutritional formula: A case report If you or your child are taking cefdinir and also consuming any iron source, the resulting stool color can be startling but is not dangerous. It resolves once the antibiotic course ends.
Other Medications That Darken Stool During Illness
Antibiotics are not the only drugs people take during illness, and some of the medications commonly used alongside antibiotics can independently darken stool. The most well-known culprit is bismuth subsalicylate, the active ingredient in products like Pepto-Bismol. When bismuth reacts with sulfur compounds produced by gut bacteria, it forms bismuth sulfide, which is intensely black. People sometimes reach for bismuth products to soothe the nausea or upset stomach that accompanies an infection or an antibiotic course, and then attribute the resulting jet-black stool to the antibiotic itself. If you are taking any bismuth-containing product, that is almost certainly the explanation for very dark or black stool.
Iron supplements also darken stool independently of the cefdinir reaction described above. Iron that is not fully absorbed in the upper intestine oxidizes as it passes through the colon, producing a dark green to black color. Since people with infections sometimes have reduced appetites and may already be taking multivitamins or iron, this effect frequently overlaps with antibiotic use. Checking the labels of everything you are taking during an illness is worth the few seconds.
When Dark Stool Actually Means Bleeding
The reason dark stool during an antibiotic course deserves attention, rather than automatic reassurance, is that genuine upper gastrointestinal bleeding also produces dark stool. When blood is exposed to stomach acid and digestive enzymes as it travels through the intestine, it turns black and tarry. Clinicians call this melena, and it has a distinctive sticky consistency and a foul, metallic smell that is noticeably different from the normal smell of stool. If you notice stool that is not just dark but tar-like and unusually foul-smelling, that is a different situation from a simple color shift caused by altered bile processing.
While antibiotics themselves rarely cause outright upper GI bleeding, the broader circumstances of being ill and medicated can raise the risk. Some people take nonsteroidal anti-inflammatory drugs like ibuprofen for pain or fever alongside their antibiotic, and those drugs are a well-established cause of stomach and duodenal ulcers that can bleed. If your dark stool appeared after combining an antibiotic with regular NSAID use, the NSAID is the more likely source of any bleeding. In hospital settings, infections and certain drugs are recognized contributors to upper gastrointestinal bleeding, particularly in patients who are already critically ill.5SpringerLink / Indian Journal of Pediatrics. Upper gastrointestinal bleeding: etiology and management
Antibiotic-Associated Hemorrhagic Colitis
A less common but more serious reason for bloody or very dark stool during antibiotic use is antibiotic-associated hemorrhagic colitis. This is actual inflammation and bleeding in the colon triggered by the disruption of normal gut flora. One recognized cause is overgrowth of a bacterium called Klebsiella oxytoca. When broad-spectrum antibiotics wipe out competing bacteria, K. oxytoca can proliferate and produce toxins that damage the colon lining.
Patients with this condition typically present with significant abdominal pain and almost always have bloody diarrhea.6PubMed. Antibiotic-associated hemorrhagic colitis caused by cytotoxin-producing Klebsiella oxytoca Reported cases have followed treatment with commonly prescribed antibiotics. Two pediatric cases developed severe abdominal pain and large amounts of bloody diarrhea after starting amoxicillin-clavulanate or amoxicillin, with K. oxytoca subsequently cultured from stool samples.7PubMed Central. Antibiotics-Associated Hemorrhagic Colitis Caused by Klebsiella oxytoca: Two Case Reports An adult case involved a 33-year-old woman who developed profuse bloody diarrhea lasting five days after a one-week course of ampicillin, with testing confirming K. oxytoca and ruling out the more commonly suspected C. difficile.8PubMed Central. A Case Report and Literature Review of Clostridium difficile Negative Antibiotic Associated Hemorrhagic Colitis Caused by Klebsiella oxytoca
The good news is that K. oxytoca hemorrhagic colitis usually resolves after the offending antibiotic is stopped. But it requires medical evaluation because the symptoms, heavy bloody diarrhea with cramping, overlap with other conditions that need different treatment. Other drugs beyond antibiotics can also cause hemorrhagic colitis, including certain anti-inflammatory drugs, anticoagulants, and some chemotherapy agents.9PubMed Central. Amoxicillin-associated hemorrhagic colitis: A case report and literature review
The C. Difficile Question
No discussion of stool changes during antibiotic use is complete without addressing Clostridioides difficile (commonly called C. diff). This bacterium is the most widely recognized infectious cause of antibiotic-associated diarrhea. Like K. oxytoca, C. difficile exploits the gap left when antibiotics disrupt normal gut flora. Once established, toxin-producing strains of C. difficile can cause illness ranging from mild watery diarrhea all the way to life-threatening colitis.10PubMed Central. Clostridium difficile-associated diarrhea in adults
C. difficile diarrhea does not always produce obviously dark or bloody stool, especially in milder cases. It more commonly starts as frequent watery stools, often with a distinctly unpleasant odor, accompanied by cramping and sometimes low-grade fever. In severe cases, though, the stool can become bloody or mucus-laden as the colon becomes increasingly inflamed. The highest risk comes with broad-spectrum antibiotics, prolonged courses, hospitalization, and older age. If you develop persistent watery diarrhea during or shortly after an antibiotic course, especially more than three loose stools a day for more than a couple of days, testing for C. difficile toxin is usually warranted.
How to Tell the Difference at Home
Distinguishing a harmless color change from something that needs medical attention is not always straightforward, but a few practical observations help:
- Consistency matters more than color alone. A stool that is darker than usual but otherwise formed and normal in texture is far less concerning than stool that is loose, watery, or frankly tarry.
- Check your other medications. If you are taking bismuth products, iron supplements, or iron-fortified formulas alongside your antibiotic, the color change is very likely chemical rather than medical.
- Smell can be informative. Melena, true blood-in-stool from upper GI bleeding, has a notably metallic, offensive smell that is different from ordinary stool odor. A simple color change from bile pigment alteration does not typically change the smell dramatically.
- Watch for accompanying symptoms. Harmless color changes from bile or iron chemistry do not come with abdominal pain, fever, or a general feeling of being unwell beyond what your underlying infection already causes. If dark stool appears alongside new cramping, dizziness, or worsening fatigue, that combination deserves prompt evaluation.
For parents of infants and toddlers on cefdinir and iron-containing formula, ask the prescribing doctor at the time of prescription whether to expect a color change. Being warned in advance eliminates most of the panic. If you were not warned and see alarming red stool, a quick call to the pediatrician’s office or nurse line can usually resolve the question without an emergency department visit, though going in is always reasonable if you are unsure.
Helping Your Gut Bacteria Recover
Since the color changes, digestive disruption, and infection vulnerability during antibiotic use all trace back to disturbed gut flora, a natural question is whether you can speed recovery. Some evidence suggests that taking probiotics during and after an antibiotic course helps. A study of healthy adults taking antibiotics found that those who also consumed a probiotic mixture had gut bacterial profiles that returned more quickly to their pre-antibiotic baseline compared to subjects who did not take probiotics. The differences were driven by changes in specific bacterial groups, including Bifidobacterium and Enterobacteriaceae.11PubMed. Probiotics to minimize the disruption of faecal microbiota in healthy subjects undergoing antibiotic therapy
This does not mean probiotics will prevent every side effect. But a faster return to normal gut flora composition could shorten the window of altered stool color and reduce the risk of opportunistic infections like C. difficile. If you decide to try a probiotic, take it at least two hours apart from your antibiotic dose to avoid the antibiotic immediately killing the probiotic organisms. Fermented foods like yogurt, kefir, and sauerkraut offer a dietary alternative, though the bacterial strains and concentrations vary widely compared to commercial probiotic supplements.
Your diet during and after an antibiotic course also matters for recovery. Fiber feeds the beneficial bacteria you want to grow back. Staying well-hydrated helps replace fluid lost to any diarrhea. And avoiding alcohol during the antibiotic course removes one more stressor from an already-disrupted digestive system.
Stool Color Changes in Children Versus Adults
Children tend to generate more alarm over stool color changes simply because caregivers are watching closely and pediatric stools are already more variable in color and consistency than adult stools. Infants and toddlers are also the age group most commonly prescribed cefdinir for ear infections and most likely to be on iron-fortified formula, making the cefdinir-iron reaction disproportionately common in this population. Three infants brought to an emergency department for “bloody diarrhea” during cefdinir courses all tested negative for actual blood in the stool, and the authors emphasized that thorough diet and drug history-taking is essential to avoid unnecessary workups and economic burden on families.3PubMed. Emergent “Bloody Diarrhea” Associated with the Use of Oral Cefdinir in Young Children: A Brief Report and Review of Literature
Adults, by contrast, are more likely to encounter dark stool from the bile pigment mechanism or from concurrent use of bismuth products and iron supplements. Adults are also at higher risk for genuine upper GI bleeding if they are combining antibiotics with NSAIDs or if they have pre-existing conditions like peptic ulcers. Older adults in particular face elevated risk for C. difficile infection, especially after hospitalization or prolonged antibiotic courses.
Regardless of age, the practical guidance is the same: note when the color change started relative to the antibiotic, inventory everything else being taken by mouth, and seek evaluation if the stool is accompanied by pain, fever, dizziness, or signs of dehydration. A simple color shift that started right after you began the antibiotic and is not accompanied by any new symptoms is overwhelmingly likely to be benign. A sudden change with worsening symptoms is not something to ride out at home.