Cefdinir 300 mg is an antibiotic used to treat mild to moderate bacterial infections of the lungs, sinuses, throat, ears, and skin. It belongs to a class of antibiotics called third-generation cephalosporins, which work by disrupting the construction of bacterial cell walls, ultimately killing the bacteria. The 300 mg capsule is the standard adult dose, typically taken once or twice a day for 5 to 10 days depending on the infection.
Infections Cefdinir 300 mg Treats
The FDA has approved cefdinir capsules for several specific infections in adults and adolescents:
- 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 (maxillary sinusitis): bacterial infection in the sinus cavities behind the cheeks
- Strep throat and tonsillitis: throat infections caused by group A strep bacteria
- Uncomplicated skin infections: bacterial infections of the skin and the tissue just below it, such as cellulitis or wound infections
In children, cefdinir is also approved for middle ear infections (acute otitis media), strep throat, and skin infections. Younger children typically receive a liquid suspension dosed by weight rather than the 300 mg capsule.
One important detail: cefdinir only works against bacterial infections. It will not treat viral illnesses like the common cold or flu, even if the symptoms feel similar to a sinus or throat infection.
How Cefdinir Works Against Bacteria
Cefdinir kills bacteria by interfering with their ability to build and maintain their outer cell walls. Without a sturdy wall, bacterial cells swell and burst. This makes cefdinir bactericidal, meaning it actively destroys bacteria rather than simply slowing their growth.
As a third-generation cephalosporin, cefdinir is effective against a broad range of bacteria. It covers many of the common culprits behind respiratory and skin infections, including strains that have developed resistance to older penicillin-type antibiotics by producing enzymes that break those drugs down. Cefdinir resists those enzymes, which is one reason doctors prescribe it when simpler antibiotics may not be reliable.
Typical Dosing Schedule
For adults and teenagers, the standard regimen is 300 mg every 12 hours or 600 mg once daily. Your prescriber chooses between these based on the type of infection. For community-acquired pneumonia, twice-daily dosing tends to be the preferred approach because it keeps drug levels more consistent in the bloodstream. Most treatment courses last 5 to 10 days.
Finishing the full course matters, even if you feel better after a few days. Stopping early can leave behind bacteria that are harder to treat if the infection returns.
Common Side Effects
Cefdinir is generally well tolerated, but digestive issues are the most frequent complaint. In clinical trials involving nearly 4,000 adult patients, about 15% experienced diarrhea and 3% reported nausea. Vaginal yeast infections occurred in roughly 4% of women, a common side effect with many antibiotics because they disrupt the normal balance of bacteria in the body.
Headache and rash are also reported occasionally but are less common than the gastrointestinal symptoms.
The Red Stool Effect
One side effect that alarms many people is reddish or rust-colored stool. This happens when cefdinir combines with iron in the digestive tract, forming a harmless reddish compound. It most often occurs in people taking iron supplements or eating iron-fortified foods (like infant formula in children). The discoloration looks alarming but causes no symptoms and tests negative for blood. It resolves on its own once you stop the antibiotic.
Iron and Antacid Interactions
Iron does more than change stool color. It also interferes with how well your body absorbs cefdinir, potentially making the antibiotic less effective. If you take iron supplements or a multivitamin containing iron, separate them from your cefdinir dose by at least 2 hours.
The same rule applies to antacids that contain magnesium or aluminum. These common over-the-counter products can block cefdinir absorption if taken at the same time. Take the antacid at least 2 hours before or after your antibiotic dose to avoid the interaction.
Cefdinir and Penicillin Allergies
Because cephalosporins and penicillins are structurally related, people with penicillin allergies sometimes worry about taking cefdinir. The risk depends on the type of allergic reaction and how closely the specific drugs are related at a chemical level. For third-generation cephalosporins like cefdinir, the cross-reactivity risk in penicillin-allergic patients is considered negligible, particularly when the penicillin that caused the reaction has a different chemical side chain than cefdinir. That said, if you’ve ever had a severe allergic reaction to any penicillin, such as throat swelling or anaphylaxis, your doctor will want to evaluate the risk before prescribing any cephalosporin.
What Cefdinir Does Not Cover
Cefdinir has real limitations. It is not effective against MRSA (methicillin-resistant staph), which is a common cause of skin infections that don’t respond to standard antibiotics. It also does not reliably treat infections caused by certain drug-resistant strains of the pneumonia-causing bacterium Streptococcus pneumoniae. For urinary tract infections, other antibiotics are typically preferred even though cefdinir has some activity against the bacteria involved.
Antibiotic resistance is a growing concern across all classes of these drugs. Cefdinir works best when used specifically for the infections it’s approved to treat, at the full prescribed dose and duration.