Took the first dose of z pack had bad reactions so stopped taking how long can side effects last for?

What sort of reactions did you have? The main side effect of azithromycin is stomach upset, sometimes with cramping and diarrhea. However, the first Z pak dose is twice as large as the remaining ones, and subsequent doses may not be so bad. Taking it with food also can prevent these problems. Don't stop without first letting your doctor know, then follow his or her advice. Good luck.