Cefdinir is a commonly prescribed antibiotic used for ear infections, sinus infections, bronchitis, and strep throat. Most people tolerate it well, but it does cause side effects, with diarrhea being the most frequent by a wide margin. In clinical trials involving nearly 4,000 patients, 15% experienced diarrhea while taking cefdinir. Other side effects range from mild stomach upset to rare but serious allergic reactions.
The Most Common Side Effects
Digestive issues top the list. In FDA clinical trials, the breakdown looked like this:
- Diarrhea: 15% of patients
- Nausea: 3%
- Headache: 2%
- Abdominal pain: 1%
Diarrhea at that rate means roughly one in seven people taking cefdinir will deal with loose stools. For most, it’s mild and clears up once the course is finished. Eating something before you take each dose can help settle your stomach, though it won’t eliminate the risk entirely.
Red or Dark Stools From Iron
One side effect catches people off guard: cefdinir can turn your stool reddish or brick-colored if you’re also taking iron supplements, iron-fortified baby formula, or multivitamins containing iron. This happens because the drug binds with iron in the gut and forms a harmless compound. It looks alarming but isn’t dangerous, and it stops once you finish the antibiotic.
To reduce this reaction (and to make sure cefdinir actually works properly), take iron supplements, multivitamins, or antacids containing aluminum or magnesium at least two hours before or after your cefdinir dose. These substances interfere with absorption, meaning less of the drug gets into your system if you take them together.
Yeast Infections and Secondary Infections
Like all antibiotics, cefdinir kills bacteria without distinguishing between harmful ones and the beneficial ones that keep your body in balance. This can lead to overgrowth of yeast, particularly in women. Vaginal yeast infections are a recognized side effect, typically showing up as itching, discharge, and irritation during or shortly after the course of treatment. Oral thrush (white patches in the mouth) can also occur, especially in young children taking the liquid suspension.
C. Diff Infection: A Serious Risk
The side effect worth knowing about is Clostridioides difficile infection, commonly called C. diff. This is a bacterial infection of the colon that antibiotics can trigger by wiping out the normal gut bacteria that usually keep C. diff in check. You’re up to 10 times more likely to develop a C. diff infection while taking an antibiotic and in the month after finishing one.
The key warning sign is diarrhea that gets worse instead of better, especially if it becomes watery, frequent (three or more times a day), or contains blood. Other symptoms include fever, stomach tenderness, loss of appetite, and nausea. This matters because ordinary antibiotic diarrhea is mild and self-limiting, while C. diff diarrhea is persistent and can become dangerous. If your diarrhea is worsening several days into treatment or starts after you’ve finished the course, that’s the pattern to take seriously.
Allergic Reactions
Cefdinir belongs to the cephalosporin class of antibiotics, which are chemically related to penicillin. If you’ve had a severe allergic reaction to penicillin, there’s a small chance you could react to cefdinir as well. Mild allergic symptoms include rash, itching, and hives. More serious reactions involve swelling of the face, lips, or throat, difficulty breathing, and dizziness, which require immediate medical attention.
In extremely rare cases, cephalosporins have been linked to Stevens-Johnson syndrome, a severe skin reaction. Early signs include fever, sore mouth and throat, fatigue, and burning eyes, typically appearing one to three days before a rash develops. The rash itself spreads rapidly, turns red or purple, and progresses to blistering of the skin and mucous membranes. This is a medical emergency, but it’s worth emphasizing how rare it is.
Effects on Kidney Function
If your kidneys don’t filter waste efficiently, cefdinir can build up in your bloodstream to higher-than-intended levels. People with significantly reduced kidney function (a filtration rate below 30 mL/min) need a lower dose to avoid this accumulation. Your prescriber should already be aware of your kidney function before writing the prescription, but if you know you have kidney disease, make sure it’s been factored into your dosing.
What to Expect During Treatment
Most cefdinir courses run 5 to 10 days. The mild side effects, particularly diarrhea and nausea, tend to show up within the first few days and often ease as your body adjusts. Finishing the full course matters even if you feel better early, because stopping short increases the chance the infection comes back and could contribute to antibiotic resistance.
Staying hydrated helps manage diarrhea. Probiotic foods or supplements may support gut health during and after antibiotic treatment, though the evidence on this is mixed. If side effects are making it hard to finish the course, that’s a conversation worth having with your prescriber, since switching to a different antibiotic is sometimes a better option than pushing through.