Omnicef (cefdinir) is an oral antibiotic used to treat mild to moderate bacterial infections of the ears, sinuses, throat, lungs, and skin. It belongs to the cephalosporin family, a class of antibiotics that kills bacteria by blocking the construction of their cell walls. Omnicef comes in capsules for adults and a liquid suspension for children, and most courses of treatment last 5 to 10 days.
Infections Omnicef Treats in Adults
For adults and adolescents, Omnicef is approved to treat five types of infections:
- Community-acquired pneumonia: a lung infection picked up outside of a hospital setting
- Acute flare-ups of chronic bronchitis: worsening symptoms in people who already have long-term bronchitis
- Acute sinus infections: bacterial sinusitis affecting the maxillary sinuses (the ones behind your cheekbones)
- Strep throat and tonsillitis: throat infections caused by group A strep bacteria
- Uncomplicated skin infections: bacterial skin infections like impetigo or mild cellulitis
The standard adult dose is 300 mg twice a day or 600 mg once a day for 5 to 10 days, depending on the infection being treated.
Infections Omnicef Treats in Children
Children are most commonly prescribed Omnicef for ear infections (acute bacterial otitis media), which is one of its primary pediatric uses. It’s also approved for strep throat and uncomplicated skin infections in kids. Pediatric dosing is weight-based, typically 14 mg per kilogram of body weight per day, up to a maximum of 600 mg daily. That dose can be split into two daily doses or given once a day. For ear infections and strep throat, treatment usually runs 5 to 10 days.
Omnicef oral suspension has a strawberry cream flavor, which makes it easier for young children to tolerate compared to some other liquid antibiotics.
Which Bacteria It Works Against
Omnicef is a third-generation cephalosporin, which means it’s effective against a broad mix of bacteria. On the gram-positive side, it targets staph (methicillin-susceptible strains only), strep pneumoniae, and group A strep. On the gram-negative side, it covers several common respiratory pathogens, including the bacteria behind most sinus and ear infections.
One advantage of Omnicef is that it works against many bacteria that produce enzymes designed to break down penicillin-type antibiotics. This makes it a useful backup when a first-line antibiotic might not be effective.
There are clear limits, though. Omnicef does not work against MRSA, enterococcus, or pseudomonas infections. It’s also ineffective against penicillin-resistant strep pneumoniae, which is important because your doctor needs to know the local resistance patterns before prescribing it for a lung or sinus infection.
Off-Label Use for Urinary Tract Infections
Although not officially FDA-approved for UTIs, Omnicef is sometimes prescribed off-label for urinary tract infections, particularly in children. Because the drug is cleared through the kidneys in its active form, it reaches the urinary tract at concentrations high enough to fight common UTI-causing bacteria like E. coli and Klebsiella. Some research has supported cefdinir as a reasonable choice for uncomplicated UTIs in adults, especially as resistance to older first-line antibiotics like trimethoprim-sulfamethoxazole continues to grow. Your prescriber may choose it when standard UTI antibiotics aren’t a good fit.
The Red Stool Side Effect
One side effect catches parents off guard: Omnicef can turn stools a bright reddish or rust color. This happens when the drug reacts with iron in the digestive tract, forming a harmless, non-absorbable complex. It’s especially common in children taking iron-fortified formula or iron supplements alongside the antibiotic. The discoloration looks alarming but is not blood and is not dangerous. It resolves on its own once the antibiotic course is finished.
Beyond this, diarrhea is the most frequent side effect, occurring in roughly 15% of people who take the drug. Rash is less common, reported in under 1% of patients in clinical trials.
Penicillin Allergy and Omnicef
If you have a penicillin allergy, you may wonder whether Omnicef is safe. As a third-generation cephalosporin, it has a low rate of cross-reactivity with penicillin, estimated at less than 1%. That’s significantly lower than older cephalosporins, which cross-react in 1% to 8% of penicillin-allergic patients. Still, if you’ve ever had a severe allergic reaction to penicillin (anaphylaxis, throat swelling, widespread hives), your doctor will weigh the risks carefully before prescribing any cephalosporin.
What Omnicef Does Not Treat
Omnicef only works against bacteria. It will not help with viral infections like the common cold, flu, or most cases of bronchitis in otherwise healthy people. Taking antibiotics for viral illness provides no benefit and contributes to antibiotic resistance. It’s also not appropriate for severe or complicated infections that require IV antibiotics, hospital-level treatment, or broader antimicrobial coverage. If your symptoms are getting worse after two to three days on Omnicef, that may signal the infection isn’t responding and a different approach is needed.