Penicillin and amoxicillin are the first-choice antibiotics for strep throat. They’ve held that position for decades because they’re effective, inexpensive, and Group A Streptococcus has never developed resistance to them. Most courses last 10 days, and you’ll typically start feeling better within a day or two of your first dose.
First-Line Options: Penicillin and Amoxicillin
Both penicillin and amoxicillin work the same way. They block the bacteria from building their cell walls, which kills actively growing strep bacteria. The CDC recommends either one as the go-to treatment for confirmed strep throat in both children and adults.
Amoxicillin is often the more practical choice, especially for kids, because it tastes better in liquid form and can be taken just once or twice a day. For children, the standard dose is based on body weight. Adults typically take 500 mg twice daily or 875 mg twice daily for 10 days. Penicillin V works just as well but usually requires more frequent dosing: adults take it two to four times per day for the same 10-day course.
There’s also an injectable option, benzathine penicillin G, given as a single shot in the clinic. This is useful when someone is unlikely to finish a full 10-day course of pills, since one dose provides the entire treatment. It’s the same antibiotic family, just delivered differently.
The 10-day course matters even though you’ll feel better long before it’s done. Stopping early doesn’t fully clear the bacteria, which increases the chance of complications and relapse.
Options If You’re Allergic to Penicillin
Penicillin allergies are common, and several alternatives exist. The choice depends on the type of allergic reaction you’ve had.
If your reaction was mild (a rash, for example, rather than throat swelling or difficulty breathing), a first-generation cephalosporin like cephalexin or cefadroxil is typically safe. These are chemically related to penicillin but different enough that most people with mild penicillin allergies tolerate them fine. Both are taken orally for 10 days.
If your reaction was severe, with hives, swelling, or breathing problems, cephalosporins are off the table. In that case, the main alternatives are:
- Azithromycin: A 5-day course, with a higher dose on day one followed by four days at a lower dose. It’s convenient, but rising resistance is a concern.
- Clarithromycin: Taken twice daily for 10 days.
- Clindamycin: Taken three times daily for 10 days.
Why Azithromycin Isn’t First Choice
Azithromycin (the well-known “Z-Pack”) is the shortest course available at just five days, which makes it popular with patients. But it has a significant drawback: strep bacteria are increasingly resistant to it. According to CDC surveillance data, roughly 1 in 3 invasive Group A strep infections now involve bacteria resistant to both erythromycin and clindamycin, the same drug class azithromycin belongs to.
That means azithromycin simply won’t work for a meaningful percentage of strep infections. It’s reserved for people who genuinely can’t take penicillin or cephalosporins, not used as a convenience alternative.
Why Strep Needs Antibiotics at All
Most sore throats are viral, and antibiotics won’t help those. But strep throat is bacterial, and leaving it untreated carries real risks. The most serious is rheumatic fever, an inflammatory condition that can damage heart valves. During historical epidemics, up to 3% of untreated strep infections led to rheumatic fever. Rates in everyday settings are lower, but appropriate antibiotic treatment prevents it in most cases.
Untreated strep can also spread to surrounding tissue, causing abscesses near the tonsils, sinus infections, or ear infections. Antibiotics dramatically reduce the odds of all of these.
How Quickly Antibiotics Work
Most people notice their sore throat improving within one to two days of starting antibiotics. Fever usually breaks within the first 24 hours. You become non-contagious surprisingly fast: within 12 hours of your first dose. That’s the standard threshold schools and workplaces use for return.
Even though symptoms improve quickly, the full course (typically 10 days for everything except azithromycin) is necessary to completely eliminate the bacteria and prevent complications. Finishing early is one of the most common reasons strep comes back.
Common Side Effects to Expect
Penicillin and amoxicillin are generally well tolerated, but they can cause nausea, vomiting, and diarrhea. These are usually mild. In children, amoxicillin occasionally causes temporary tooth discoloration (yellowish or brown), which goes away on its own.
Signs of an allergic reaction are different from side effects and need immediate attention. These include a rash or hives, swelling of the face or throat, difficulty breathing, or skin blistering. Severe diarrhea that is watery or bloody, sometimes appearing weeks after finishing the course, can signal an intestinal infection triggered by the antibiotic disrupting gut bacteria.
Getting the Right Diagnosis First
Antibiotics should only be prescribed after strep is confirmed, either through a rapid antigen test (the quick swab that gives results in minutes) or a throat culture. Doctors use a clinical scoring system to decide who should be tested in the first place, based on factors like fever, swollen tonsils, swollen lymph nodes in the neck, and the absence of a cough. A sore throat with a cough and runny nose is far more likely to be viral, and antibiotics won’t help.
If your rapid test comes back negative but your doctor still suspects strep, a throat culture (which takes one to two days) can catch infections the rapid test misses, particularly in children.