Penicillin and amoxicillin are the top choices for treating strep throat, and they have been for decades. The CDC lists both as the recommended first-line antibiotics for group A strep pharyngitis. The standard course is 10 days, and most people start feeling better within the first two to three days.
Why Penicillin and Amoxicillin Come First
These two antibiotics work by breaking down the bacterial cell wall. They bind to proteins inside the bacteria that are essential for building and maintaining that wall, which causes the bacteria to burst and die. This makes them highly effective at clearing strep infections rather than just slowing bacterial growth.
One major reason penicillin and amoxicillin remain the go-to options: group A strep has never developed resistance to them. There has never been a single documented case of a strep A strain that could withstand penicillin or related antibiotics in the same class. That’s remarkable, and it means these drugs work reliably every time for this particular infection.
Amoxicillin tends to be prescribed more often for children because it comes in a liquid form that tastes better, and it can be given just once or twice a day. Penicillin V, the oral tablet form, is typically taken two to four times daily for adolescents and adults. Both are taken for a full 10-day course.
What the 10-Day Course Looks Like
For adults, penicillin V is usually prescribed as 500 mg twice daily for 10 days. Amoxicillin can be taken as a single daily dose of up to 1,000 mg or split into two doses of up to 500 mg each. Children receive weight-based doses, generally 50 mg per kilogram of body weight per day for amoxicillin.
There is also a one-shot option. A single intramuscular injection of benzathine penicillin G eliminates the need to take pills for 10 days. This can be useful if finishing a full course of oral antibiotics is a concern. The dose depends on weight: children under about 60 pounds get a smaller dose, while older kids and adults receive a larger one.
You’ll likely notice your throat feeling significantly better within 48 to 72 hours of starting antibiotics. That improvement can be tempting to interpret as a sign that you’re done, but stopping early is a common mistake. The full 10 days are needed to completely wipe out the bacteria and reduce the risk of complications, even though symptoms improve much sooner. Most people are considered no longer contagious after about 12 to 24 hours on antibiotics, which is the general guideline for returning to work or school.
Options If You’re Allergic to Penicillin
If you have a penicillin allergy, the alternatives depend on how severe that allergy is. For people whose reaction was mild (a rash, for example, rather than throat swelling or difficulty breathing), a first-generation cephalosporin is often used. Cephalosporins are in a related antibiotic family but are generally safe for people with non-severe penicillin allergies. Like penicillin, group A strep has no documented resistance to cephalosporins.
For people with a history of a serious allergic reaction to penicillin, the options shift to different antibiotic classes entirely. Macrolide antibiotics like azithromycin or clindamycin are sometimes prescribed. However, these come with an important caveat: strep bacteria can resist macrolides. Surveillance data from Canada shows that roughly 9 to 15 percent of invasive group A strep samples tested between 2018 and 2022 were resistant to erythromycin, the macrolide used as a benchmark for this class. That means these alternatives fail more often than penicillin-based options, making them a backup rather than a first choice.
Why Antibiotics Matter for Strep Specifically
Strep throat is not like a typical sore throat caused by a virus. Left untreated, group A strep can trigger complications that go well beyond the throat. The most serious of these is rheumatic fever, an inflammatory condition that can permanently damage heart valves. Antibiotics effectively prevent this, even when started as late as nine days after symptoms begin, according to the American Heart Association.
Beyond rheumatic fever, untreated strep can lead to peritonsillar abscesses (a painful collection of pus near the tonsils), kidney inflammation, and spread of the infection to surrounding tissue. Treating with the right antibiotic shortens how long you feel sick, cuts down on transmission to people around you, and prevents these complications from developing in the first place.
Getting the Right Diagnosis First
Antibiotics only help if strep bacteria are actually causing your sore throat, and most sore throats in adults are viral. A rapid strep test, done with a quick throat swab at a clinic, gives results in minutes. If that test comes back negative but strep is still suspected (especially in children), a throat culture may follow. Starting antibiotics before confirming strep isn’t recommended because unnecessary antibiotic use contributes to resistance problems with other bacteria, and it won’t do anything for a viral infection.
Classic signs that point toward strep rather than a virus include a sudden, severe sore throat without a cough, swollen and tender lymph nodes in the front of the neck, fever, and white patches or redness at the back of the throat. A cough, runny nose, and hoarseness are actually more suggestive of a viral cause, which antibiotics won’t help.