Penicillin and amoxicillin are the first-line antibiotics for strep throat, and they have been for decades. Group A Streptococcus, the bacterium behind strep throat, has never developed resistance to penicillin, making it one of the most reliable antibiotic-bacteria matchups in medicine. Treatment lasts 10 days, and most people start feeling better within one to two days of their first dose.
Penicillin and Amoxicillin: The Standard Treatment
Penicillin V (the oral tablet form) and amoxicillin are both equally effective against strep throat. They work by disrupting the bacterial cell wall during growth, which kills the bacteria rather than simply slowing them down. Your doctor will typically choose between the two based on age, convenience, and taste (amoxicillin comes in a liquid form that children tolerate better).
For adults and adolescents, penicillin V is usually prescribed at 500 mg twice daily for 10 days. Children typically take 250 mg two or three times daily for the same duration. Amoxicillin is often preferred for kids because it can be given once daily, which makes it easier for parents to keep on schedule. The CDC lists a standard dose of up to 1,000 mg once daily or 500 mg twice daily for amoxicillin.
The full 10-day course matters even though you’ll feel better long before it’s finished. Stopping early doesn’t reliably clear the bacteria from your throat, which increases the risk of the infection coming back or, more importantly, of a rare but serious complication called rheumatic fever. Penicillin effectively prevents rheumatic fever even when treatment doesn’t begin until nine days after symptoms start, so there’s no need to panic if your diagnosis is slightly delayed.
Options If You’re Allergic to Penicillin
If you have a penicillin allergy, your doctor has a few alternatives. The most common substitutes include cephalexin (a first-generation cephalosporin), azithromycin (the well-known “Z-Pack”), and clindamycin. However, these are considered second-line options for good reason.
Azithromycin in particular has a growing resistance problem. Studies of Group A Strep isolates have found macrolide resistance rates (the drug class azithromycin belongs to) as high as 73 to 78 percent in some populations. Resistance to clindamycin has followed a similar trend. This doesn’t mean these drugs never work, but it does mean they fail more often than penicillin, which maintains a perfect track record. If you’ve been told you’re penicillin-allergic, it may be worth discussing allergy testing with your doctor. Many people who were labeled allergic as children turn out to tolerate penicillin without issues.
What to Expect After Starting Treatment
Most people notice significant improvement within 24 to 48 hours of their first dose. Fever usually breaks first, followed by gradual relief of the sore throat. If you don’t feel any better after 48 hours on antibiotics, contact your doctor. That could signal a different diagnosis, a resistant strain, or another issue that needs a second look.
You’re considered no longer contagious after 24 hours on antibiotics, as long as your fever has resolved. That’s the standard threshold for returning to work, school, or daycare. Before treatment, strep is highly contagious through respiratory droplets and direct contact, so those first 24 hours on medication are a good time to stay home and rest.
Why the Full 10 Days Matters
The 10-day course serves two purposes. The first is clearing the infection completely. Strep bacteria can linger in your throat even after symptoms disappear, and a shortened course gives surviving bacteria a chance to rebound. The second, more critical purpose is preventing rheumatic fever, a condition where the immune system attacks the heart, joints, and nervous system weeks after an untreated or inadequately treated strep infection. Rheumatic fever is uncommon in the United States today largely because antibiotic treatment is so widespread, but it remains a real risk when strep goes untreated.
Other potential complications of untreated strep include peritonsillar abscess (a painful pocket of pus near the tonsils) and post-streptococcal kidney inflammation. These are all preventable with a straightforward course of antibiotics, which is why doctors test for strep rather than assuming a sore throat is viral.
When Strep Keeps Coming Back
Some people, especially children, deal with recurrent strep infections. This can happen for several reasons: re-exposure from a household contact who carries the bacteria, incomplete courses of antibiotics, or simply being in environments like schools where strep circulates easily. If strep returns shortly after finishing treatment, your doctor may try a different antibiotic or a combination approach, such as amoxicillin paired with clavulanate, which broadens the drug’s effectiveness against bacteria that might be shielding the strep from treatment.
For people with frequent recurrences (generally defined as multiple confirmed episodes within a year), tonsillectomy becomes part of the conversation. It’s not a first resort, but it can significantly reduce the frequency of strep infections in people whose tonsils seem to harbor the bacteria repeatedly.