How long would a reaction take from azythromicin?

Azithromycin : has a half life of almost 4 days, so once u take one dose, it will be in your system for at least that long. With adverse reactions to drugs (allergies), these can occur at any time during therapy-- Once your body realizes "hey I don't like this stuff". The good news is azithromycin rarely causes serious side effects, so hopefully u are doing OK...