Penicillin and amoxicillin are the first-line antibiotics for strep throat. They remain the top choices because the bacteria that cause strep, Group A Streptococcus, have never developed resistance to them. Most courses last 10 days, and you should start feeling better within one to two days of your first dose.
Why Penicillin and Amoxicillin Come First
Group A Strep is one of the rare bacteria that has stayed fully susceptible to penicillin despite decades of use. That makes penicillin and its close relative amoxicillin the safest, most effective, and cheapest options. Amoxicillin is often preferred for children because it comes in a chewable tablet and a flavored liquid suspension that tastes better than penicillin. Both work equally well in adults.
The standard course is 10 days. That length is important: it’s not just about clearing the infection, but about preventing a rare complication called rheumatic fever, which can damage heart valves. Even though you’ll feel significantly better after a day or two, stopping early leaves bacteria behind and raises the risk of complications or a rebound infection.
Options If You’re Allergic to Penicillin
A penicillin allergy doesn’t leave you without good choices. The CDC lists several alternatives, and which one your provider picks depends on the type of allergic reaction you’ve had.
If your allergy was mild (a rash, for instance, rather than throat swelling or difficulty breathing), certain cephalosporin antibiotics like cephalexin and cefadroxil are safe options. These are chemically related to penicillin but different enough that most people with mild penicillin allergies tolerate them without problems. Both are taken for 10 days.
If your allergy was severe, involving hives, swelling, or anaphylaxis, cephalosporins are off the table. In that case, the alternatives include:
- Clindamycin: taken three times daily for 10 days
- Azithromycin (Z-pack): a 5-day course, which makes it the shortest option
- Clarithromycin: taken twice daily for 10 days
Why Azithromycin Isn’t the First Choice
Azithromycin is probably the antibiotic people request most often for strep throat. A 5-day course sounds a lot easier than a 10-day one. But there’s a significant downside: resistance. Among invasive Group A Strep samples tested in 2023, 27% were resistant to macrolide antibiotics like azithromycin. That means roughly 1 in 4 strep infections may not respond to it at all. Clindamycin resistance runs at similar levels, around 26%.
Penicillin and amoxicillin have no such problem. Their resistance rate remains at zero. That’s why azithromycin and clindamycin are reserved for people who genuinely can’t take penicillin-based drugs rather than prescribed as a convenience.
How Quickly You’ll Feel Better
Most people notice real improvement within one to two days of starting antibiotics. Fever typically breaks first, followed by less throat pain and easier swallowing. If you’re not improving at all after two days on antibiotics, contact your provider. It could mean the infection isn’t strep (rapid tests occasionally give false positives), or that the specific antibiotic isn’t working.
You stop being contagious about 12 hours after your first dose. That’s the standard threshold schools and workplaces use for return. Before those 12 hours are up, you can still spread the bacteria through coughing, sneezing, or sharing food and drinks.
Why Finishing the Full Course Matters
Ten days of antibiotics feels like a lot when your throat stops hurting on day two. But the bacteria that cause strep throat aren’t fully eliminated that quickly. Stopping early allows surviving bacteria to repopulate, which can lead to a relapse that’s harder to treat and potentially exposes you to complications like rheumatic fever or kidney inflammation.
Setting a daily phone alarm or keeping the bottle next to something you use every morning (your toothbrush, your coffee maker) can help you stay on track through the second half of the course, when you feel fine and it’s easy to forget. If you do miss a dose, take it as soon as you remember and continue the regular schedule from there.