Azithromycin in a single dose of 2 g is one of the back-up recommendations for gonorrhea treatment, if the treatments of choice (e.g. ceftriaxone, cefixime) are not available or if the infected person may be allergic to them. It is very reliable and has the advantage of also being effective against chlamydia -- which actually requires only a 1 g dose.
Except for rare allergic reactions, the only side effects of azithromycin are related to gastrointestinal intolerance. Some people get upper abdominal pain or nausea within an hour. Vomiting is uncommon, but if it happens very quickly (within 10-15 min) and the drug comes back up, of course treatment may not be effective. In addition, a fair number of treated persons get cramps and diarrhea the next day. All these problems are pretty much harmless, just uncomfortable for a little while.
It takes several days for symptoms to clear up. Have you been diagnosing and treating yourself? Mistake. I can't help as much. And not knowing what you have, if anything, also makes it harder -- and I will not try to guess at your diagnosis. See a health professional and report what s/he says; until then I wont' have any other comments or advice.
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