Penicillin and amoxicillin are the recommended antibiotics for strep throat. They’ve been the first-line treatment for decades because the bacteria that cause strep (Group A Streptococcus) have never developed resistance to them. The standard course lasts 10 days, and most people start feeling better within one to two days of their first dose.
Why Penicillin and Amoxicillin Come First
The CDC lists penicillin or amoxicillin as the antibiotics of choice for strep throat. Both are equally effective at clearing the infection, but amoxicillin is more commonly prescribed for children because it comes in a chewable tablet and a flavored liquid suspension that’s easier to take. Amoxicillin also has the advantage of once-daily dosing for kids, while penicillin typically needs to be taken two to four times a day.
For adults and teens, penicillin V is usually prescribed at 500 mg twice daily for 10 days. Children take a lower dose of 250 mg two or three times daily. Amoxicillin dosing for children is based on weight. Your doctor or pharmacist will calculate the right amount, but the total course is the same 10 days regardless of which drug you’re prescribed.
Group A Strep remains fully susceptible to penicillin, which is unusual. Most bacteria develop resistance to antibiotics over time, but after more than 70 years of use, penicillin still works against every strain of this particular bacterium. That’s a major reason it stays at the top of the list.
If You’re Allergic to Penicillin
For people with a penicillin allergy, doctors typically turn to one of two antibiotic classes: cephalosporins or macrolides. If your allergy is mild (a rash, for example, rather than a severe reaction like throat swelling), a first-generation cephalosporin is often used because it’s closely related to penicillin and works just as well against strep.
If your allergy is severe, macrolide antibiotics like azithromycin or erythromycin are the usual alternatives. However, these carry a caveat. Canadian surveillance data from 2018 to 2022 found that roughly 9% to 15% of Group A Strep samples were resistant to erythromycin, and resistance to one macrolide generally means resistance to others in the same class. That means azithromycin won’t work for every patient, which is one more reason penicillin and amoxicillin remain the preferred options when allergy isn’t a concern.
The Injectable Option
There’s also a one-and-done option: a single injection of long-acting penicillin (penicillin G benzathine), given by a nurse or doctor into a large muscle, usually the thigh or buttock. This is sometimes chosen when there’s concern that a patient won’t complete the full 10-day oral course, or when someone is vomiting and can’t keep pills down. One shot delivers a sustained level of the antibiotic over days, eliminating the need to remember multiple daily doses. The trade-off is that the injection itself can be painful.
Why the Full 10 Days Matter
You’ll likely feel significantly better within 48 hours of starting antibiotics, and your fever may break even sooner. If you don’t notice any improvement after two full days, that’s a reason to call your doctor. But feeling better is not the same as being cured. The 10-day course exists primarily to prevent rheumatic fever, a serious inflammatory condition that can damage heart valves. It typically develops about 20 days after the initial strep infection in untreated cases.
There is some debate about whether shorter courses could work. A Cochrane review found that three to six days of antibiotics cleared the infection comparably to the standard 10 days in children. But the studies weren’t large enough to draw firm conclusions about whether shorter courses prevent rheumatic fever as effectively. Until that evidence is stronger, the 10-day course remains the standard recommendation, particularly because rheumatic fever, while rare, causes permanent damage when it does occur.
How Quickly You Stop Being Contagious
One of the practical benefits of antibiotics is how fast they reduce the spread. You’re generally no longer contagious within 12 hours of taking your first dose. Schools and daycares follow this same guideline, allowing children to return after 12 hours on antibiotics as long as they’re feeling well enough. Without antibiotics, strep remains contagious for two to three weeks.
What Happens With Recurring Infections
Some people, especially children, seem to get strep throat repeatedly. The first-line treatment doesn’t change for recurrent cases. Penicillin and amoxicillin are still the go-to antibiotics each time. If the same strain keeps coming back despite proper treatment, your doctor may investigate whether someone in the household is an asymptomatic carrier, meaning they carry the bacteria in their throat without symptoms and keep reintroducing it. In cases of very frequent recurrence (typically seven or more episodes in a single year, or five per year over two years), tonsillectomy becomes a conversation worth having.
The key thing to remember: strep throat is one of the most straightforward bacterial infections to treat. The antibiotics are cheap, effective, and well tolerated. The bacteria haven’t outsmarted them yet. What matters most is confirming the diagnosis with a rapid test or throat culture, starting treatment promptly, and finishing the full course even after you feel fine.