Penicillin and amoxicillin are the first-choice antibiotics for strep throat. Both are equally effective at killing group A strep bacteria, and neither has ever developed resistance to the other. The standard course lasts 10 days, and most people start feeling better within 2 to 3 days of starting treatment.
Why Amoxicillin and Penicillin Are First Choice
Group A strep remains universally susceptible to penicillin. Unlike many other bacteria, it has never developed resistance, which makes penicillin-based antibiotics the most reliable 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 a day instead of multiple times.
Amoxicillin is typically taken once daily (up to 1,000 mg) or split into two doses for 10 days. Penicillin V, the pill form of penicillin, is usually taken twice daily for adults at 500 mg per dose, also for 10 days. Children receive weight-based doses of either medication. The full 10 days matters even if you feel fine after a few days, because stopping early increases the risk of the infection coming back and of complications like rheumatic fever.
The Single-Shot Option
If finishing a 10-day course of pills sounds unrealistic, there’s a one-and-done alternative: a single penicillin injection given in the muscle. This delivers the antibiotic in a slow-release form that stays in your system long enough to clear the infection without any follow-up doses. It’s commonly used for children or anyone who might struggle with medication adherence. The injection can sting, but it eliminates any guesswork about completing treatment.
Alternatives If You’re Allergic to Penicillin
A penicillin allergy changes the playbook. The most common alternatives fall into a few categories:
- Cephalosporins (like cephalexin) work well for strep and are safe for most people with penicillin allergies. First-generation cephalosporins carry a small cross-reactivity risk of 1% to 8%, so your provider may want to know the severity of your allergy before prescribing one. Third-generation cephalosporins have less than 1% cross-reactivity.
- Azithromycin (the “Z-pack”) is sometimes prescribed, but rising resistance is a concern. Recent UK surveillance data found that 19% of group A strep isolates were resistant to erythromycin, a closely related antibiotic in the same class. Certain strains show resistance rates above 90%. This means a Z-pack may simply not work for a growing number of strep infections.
- Clindamycin is another option, though it carries a higher risk of digestive side effects, including severe diarrhea from disrupting gut bacteria.
If you’ve been told you’re allergic to penicillin, it’s worth knowing that many people who were labeled allergic as children can actually tolerate it. Allergy testing can clarify this and potentially open up the most effective treatment option.
What Side Effects to Expect
Amoxicillin and penicillin are generally well tolerated, but a 10-day course does give your gut flora a shake-up. The most common side effects are nausea, vomiting, and diarrhea. These are usually mild and manageable. Some children develop a temporary yellowish or brownish discoloration of their teeth, which resolves after treatment.
Serious reactions are uncommon but worth recognizing: a rash with skin blistering or peeling, swelling of the face or throat, or difficulty breathing are signs of a true allergic reaction that needs immediate attention. Severe watery or bloody diarrhea, sometimes with fever and stomach cramps, can signal a gut infection caused by antibiotic disruption. This can occasionally appear up to two months after finishing the course.
How Quickly You’ll Feel Better
Most people notice improvement within 48 to 72 hours of starting antibiotics. Fever typically breaks first, followed by reduced throat pain. You stop being contagious after the first 24 to 48 hours of treatment, which is why schools and workplaces generally allow you to return after a full day on antibiotics.
If your symptoms haven’t improved at all after 3 to 4 days, or if they come back shortly after finishing the course, you may need to be retested. Recurrence sometimes happens because a household member is carrying the bacteria and reinfecting you, or because the original infection wasn’t fully cleared. In those cases, your provider may prescribe a different antibiotic or the penicillin injection to ensure complete treatment.
Why the Full Course Matters
Treating strep throat isn’t just about feeling better. Untreated or undertreated strep can lead to rheumatic fever, a condition where the immune system attacks the heart, joints, and nervous system. Rheumatic fever is rare in developed countries precisely because antibiotics are so widely used for strep, but it still occurs. Completing the full 10-day course, or getting the single injection, is the primary way to prevent it. Antibiotics also reduce the risk of the infection spreading to the sinuses, ears, or bloodstream, and they shorten the window in which you can pass strep to others.