Penicillin and amoxicillin are the first-line antibiotics for strep throat. They’ve been the standard treatment for decades, and group A strep bacteria have never developed resistance to either one. A full course lasts 10 days, and most people start feeling better within one to two days of their first dose.
First-Line Options: Penicillin and Amoxicillin
Both penicillin V (the oral tablet form) and amoxicillin work equally well against strep. Amoxicillin is prescribed more often for children because it tastes better in liquid form and can be given just once or twice a day, while penicillin V sometimes requires more frequent doses.
For adults and teens, penicillin V is typically taken as 500 mg twice daily for 10 days. Children usually take 250 mg two or three times daily for the same duration. Amoxicillin dosing for children is based on weight, and either option is taken for a full 10 days regardless of how quickly symptoms improve.
There is also a single-shot option. An injection of long-acting penicillin G delivers one dose that covers the entire treatment course. This is sometimes used when there’s concern about someone finishing a full 10-day oral course, or when a person is vomiting and can’t keep pills down.
If You’re Allergic to Penicillin
For people with a penicillin allergy, the alternatives get more complicated. Some doctors prescribe a type of antibiotic called a cephalosporin (like cephalexin), which is closely related to penicillin but tolerated by many people with mild penicillin allergies. If your allergy was severe, involving swelling, difficulty breathing, or anaphylaxis, cephalosporins are usually avoided too.
Azithromycin (the well-known “Z-pack”) and clindamycin have historically been backup options, but resistance is now a real concern. About 1 in 3 invasive group A strep infections are caused by bacteria resistant to these antibiotics. That doesn’t mean they never work for strep throat, but your doctor may want to choose carefully or order a sensitivity test. Penicillin and amoxicillin remain completely effective, with zero resistance reported.
What to Expect After Starting Treatment
Most people notice their fever dropping and throat pain easing within the first 24 to 48 hours. You’re considered no longer contagious after just 12 hours on antibiotics, which is the standard threshold for returning to school or work. If you don’t have a fever, many guidelines allow you to go back to normal activities 24 hours after your first dose.
Feeling better quickly can make it tempting to stop the antibiotics early. Don’t. Symptom improvement doesn’t mean the bacteria are fully cleared. The 10-day course exists for a specific reason: it dramatically reduces the risk of a serious complication called acute rheumatic fever, which can damage the heart valves.
Why the Full 10 Days Matter
Without antibiotics, roughly 2.4% of people with strep throat develop acute rheumatic fever within one to two months. With a full course of penicillin, that drops to about 0.35%, an 80% reduction in risk. That might sound like small numbers, but rheumatic fever can cause permanent heart damage, joint inflammation, and neurological problems. It’s entirely preventable with proper treatment.
Antibiotics also reduce the chance of other complications like peritonsillar abscess (a painful pocket of pus near the tonsils) and the spread of infection to the sinuses or ears. And completing your course helps protect the people around you. Strep spreads easily through respiratory droplets, and cutting treatment short means you could remain infectious longer than expected.
Strep in Children vs. Adults
Children between 5 and 15 are the most common age group for strep throat, and treatment is essentially the same as for adults: penicillin or amoxicillin for 10 days. The main difference is dosing. Children’s amoxicillin doses are calculated by body weight, typically 50 mg per kilogram once daily or 25 mg per kilogram twice daily, up to the adult maximum. Your pharmacist will calculate the exact amount based on your child’s weight.
For younger children who struggle with swallowing pills, both amoxicillin and penicillin V come in liquid suspensions. The single penicillin injection is another option if a child is unlikely to tolerate 10 days of oral medication. Kids under about 60 pounds receive a smaller injection dose, while older children get the same dose as adults.
When Antibiotics Don’t Seem to Work
If your symptoms haven’t improved at all after 48 to 72 hours on antibiotics, contact your doctor. This could mean the diagnosis was wrong (many sore throats are viral and won’t respond to antibiotics), the strain is resistant to the prescribed drug, or there’s a secondary issue like an abscess forming. A repeat rapid strep test or throat culture can help sort this out.
Some people get recurrent strep infections, meaning they keep testing positive every few weeks or months. This is different from treatment failure and may involve re-exposure from a family member or close contact who’s carrying the bacteria without symptoms. In these cases, a doctor might try a different antibiotic regimen or test household contacts to identify the source.