On antibiotics, strep stops being contagious after about 12 to 24 hours (with fever gone). Without treatment, it can spread for 2 to 3 weeks even as you feel better.
That is why the return-to-school or work rule is the day after antibiotics start, not the day symptoms ease. While contagious: no shared cups, utensils, or towels, cover coughs, wash hands. Two rules people break: finish the full antibiotic course even when the throat feels fine by day 2 (stopping early invites rheumatic fever and recurrence), and get tested rather than assuming, since most adult sore throats are viral and need no antibiotics at all. Household hygiene: swap your toothbrush after a day or two on treatment, and keep kids home until they clear the 24-hour-plus-no-fever bar. Not better after 48 hours on antibiotics? Call the prescriber; that pattern needs a second look.
What to do
Common questions
How long am I contagious with strep throat?
With antibiotics: you stop being contagious roughly 12 to 24 hours after the first dose, provided your fever is also gone, which is why schools and workplaces generally let you return the day after starting treatment. Without antibiotics, you can keep spreading strep for 2 to 3 weeks, even as symptoms fade, which is the real reason testing and treatment matter beyond just feeling better faster. How it spreads: respiratory droplets and shared surfaces, so while you are contagious, do not share cups, utensils, or towels, cover coughs, and wash hands often. Practical notes: the sore throat usually improves within 48 hours of antibiotics, but finish the full course anyway, because stopping early risks rheumatic fever and recurrence. If you never got a test, do not assume: most adult sore throats are viral, antibiotics do nothing for those, and the contagiousness math is different. And the household math: replace your toothbrush after you have been on antibiotics for a day or two, and keep kids home from school until they meet the 24-hours-on-antibiotics-plus-no-fever bar. If your "strep" is not improving after 48 hours on antibiotics, call the prescriber, because that pattern suggests something else is going on.