Cefdinir: what it treats, how to take it, side effects
Last updated September 3, 2026.
Cefdinir is a cephalosporin antibiotic - it kills the bacteria behind common respiratory and skin infections. It is a workhorse for ear infections, sinus infections, and strep, with two quirks people always ask about: red stools and a birth-control interaction.
What does it treat?
Bacterial infections including bronchitis, pneumonia, and infections of the skin, ears, sinuses, throat, and tonsils. It kills bacteria; it does nothing for colds, flu, or other viruses.
How do you take it?
With or without food, every 12 or 24 hours depending on what is being treated, for 5 to 10 days - same times every day. Capsules come as 300 mg; shake the suspension before each use. Two spacing rules: antacids containing magnesium or aluminum, iron supplements, and iron-containing multivitamins go 2 hours before or 2 hours after cefdinir. And the famous quirk: it can turn stools reddish, especially alongside iron - harmless, but nobody warns you. Finish the whole course even when you feel better. Missed dose: take it when you remember unless the next one is close; never double up.
Side effects to know
- Common: vomiting, nausea, stomach pain, diarrhea, headache, vaginal itching, and reddish-colored stools.
- Serious - call your doctor immediately or get emergency care: rash, hives, swelling of the face, throat, tongue, lips, or eyes, and difficulty breathing or swallowing.
Who should not take it?
Anyone allergic to cefdinir or other cephalosporins - and penicillin allergy goes on the list too, since cross-reactions exist. Flag gastrointestinal disease, especially colitis, and kidney disease. If you have diabetes, the suspension contains sugar. And the birth-control rule: cefdinir may decrease the effectiveness of hormonal contraceptives - pills, patches, rings, and injections - so a backup method for the course. If you become pregnant, call your doctor.
When is it an emergency?
Trouble breathing or swallowing, or swelling of the face, tongue, or throat, is a 911 call. For an overdose, call Poison Control at 1-800-222-1222. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper.
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Common questions
Why is my poop red on cefdinir?
Almost certainly the drug, not blood. MedlinePlus lists reddish-colored stools among cefdinir's common effects - it is a known, harmless interaction, especially with iron in your diet or supplements. The versions that are not harmless - watery or bloody stools with cramps or fever - come with symptoms you cannot miss and mean calling your doctor.
Why do I have to separate cefdinir from my iron supplement?
Iron binds it in the gut and blocks absorption. MedlinePlus says antacids with magnesium or aluminum, iron supplements, and iron-containing multivitamins go 2 hours before or 2 hours after cefdinir. Same window, three common products - check your multivitamin label.
Does cefdinir affect birth control?
It can. MedlinePlus says cefdinir may decrease the effectiveness of hormonal contraceptives - pills, patches, rings, and injections - and that you will need another method while taking it. Sort the backup before the first dose.
I am allergic to penicillin - can I take cefdinir?
Tell your doctor before the first dose, not after. Cefdinir is a cephalosporin, a cousin of penicillin, and MedlinePlus says to disclose allergy to any cephalosporin or to penicillin antibiotics. Many penicillin-allergic people take cephalosporins safely, but that call belongs to your prescriber with your specific reaction history.
Why is the suspension a problem for diabetics?
It is sweetened with sucrose. MedlinePlus says if you have diabetes, you should know the cefdinir suspension contains sugar - relevant to glucose control and worth factoring into your monitoring during the course.
What happens if I miss a dose of cefdinir?
Take it as soon as you remember, unless it is almost time for the next one - then skip it and never double up. With a 5-to-10-day course, the finish line is close - the complete course is what makes the infection stay gone.