Penicillin and amoxicillin are the first-line antibiotics for strep throat. They’ve held that position for decades because Group A Streptococcus, the bacterium behind strep throat, has never developed resistance to penicillin-type drugs. Most people start feeling better within one to two days of their first dose, and the full course typically lasts 10 days.
Why Penicillin and Amoxicillin Come First
Penicillin and amoxicillin work by blocking the strep bacterium from building its protective outer wall. Without that wall, the bacteria can’t survive. This mechanism is remarkably effective against Group A Strep, and unlike many other bacterial infections, resistance to penicillin has simply not emerged in this species. That makes penicillin-based antibiotics the most reliable, narrowly targeted option available.
Amoxicillin is often preferred in practice, especially for children, because it tastes better in liquid form and can be taken once or twice daily rather than multiple times a day. Both drugs are taken for 10 days. Penicillin can also be given as a single injection, which eliminates the need to remember daily doses altogether.
Options If You’re Allergic to Penicillin
If you have a penicillin allergy, several alternatives work well. The CDC recommends these options, all taken for 10 days unless noted otherwise:
- Cephalexin: A cephalosporin antibiotic taken twice daily for 10 days. It’s closely related to penicillin, so it’s only appropriate if your allergy was mild (like a rash), not a severe reaction involving throat swelling or difficulty breathing.
- Cefadroxil: Another cephalosporin, taken once daily for 10 days. The same caution about severe penicillin allergies applies.
- Clindamycin: Taken three times daily for 10 days. Safe for people with any type of penicillin allergy.
- Azithromycin: A five-day course, which is shorter than other options. You take a larger dose on day one and a smaller dose for the next four days.
- Clarithromycin: Taken twice daily for 10 days.
Your prescriber will choose among these based on the nature of your allergy and local resistance patterns.
The Problem With Azithromycin
Azithromycin (the “Z-Pack”) is probably the antibiotic people request most often because of its short, convenient course. But there’s a growing issue: Group A Strep is increasingly resistant to it. Among invasive strep isolates tested in 2023, 27% were resistant to macrolide antibiotics like azithromycin. Clindamycin resistance was similarly high at 26%.
That doesn’t mean these drugs never work, but it does mean they fail more often than penicillin or amoxicillin. If you’re not allergic to penicillin, azithromycin is not the better choice simply because the course is shorter. Penicillin and amoxicillin remain effective in virtually every case.
What to Expect During Treatment
Most people notice significant improvement within one to two days of starting antibiotics. The sore throat eases, fever drops, and swallowing becomes less painful. You’re generally considered no longer contagious after about 12 to 24 hours on antibiotics, which is the typical threshold for returning to work or school.
Feeling better quickly creates a temptation to stop taking the medication early. Finishing the full course matters. Stopping too soon can leave enough bacteria alive to cause a relapse or, more importantly, to trigger complications that antibiotics are specifically meant to prevent.
Why Antibiotics Matter Beyond Symptom Relief
Strep throat often resolves on its own within a week, which raises a fair question: why bother with antibiotics at all? The answer is complications. Untreated strep can lead to acute rheumatic fever, a condition where the immune system attacks the heart valves. During historical epidemics, as many as 3% of untreated strep infections progressed to rheumatic fever. That rate is lower in everyday settings, but the stakes are high enough that treatment is standard.
Antibiotics also reduce the risk of peritonsillar abscess (a painful collection of pus near the tonsils) and prevent the infection from spreading to the sinuses, ears, or bloodstream. Shortening the contagious window protects the people around you, particularly young children and anyone with a weakened immune system.
Finishing the Full 10 Days
The standard course for most strep antibiotics is 10 days. Azithromycin’s five-day course is the exception. Even though you’ll likely feel fine by day three or four, the remaining doses are doing important work: fully clearing the bacteria from your throat and reducing the chance of a rebound infection. Stopping early is the single most common reason people end up needing a second round of treatment. If you’re having side effects that make finishing difficult, call your prescriber to discuss switching to a different antibiotic rather than simply stopping.