A single, 1500 mg dose of azithromycin (zithromax) would be effective in treating a confirmed or presumed genital chlamydial infection. This dosage would also be used to treat someone with gonorrhea, but only in addition to another antibiotic, like cefixime or ceftriaxone. The azithromycin is not effective against gonorrhea, but since so many people who have gonorrhea also can have chlamydia, it is recommended that individuals in whom this is suspected should receive two antibiotic regimens.
Azithromycin is not recommended or effective for the treatment of syphilis. Instead, penicillin, doxycycline, and tetracycline are recommended, with different dosing schedules and follow-up depending on the type of syphilis (the disease can present in a few different stages).
So really, the zithromax would only potentially be effective for genital chlamydial infection, and for the other two entities, other antiobiotics would be recommended and required.
If you were potentially exposed to any of these STDs, then I recommend that you get tested. Treatment is available and well-tolerated for all of them. So call your physician or a clinic, and be tested. Better safe than sorry.
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