My first response addresses the premise of your question, that you should take any antibiotic at all after your sexual encounters. This is a bit controversial, but public health experts recommend this in only when the risk of STI is especially high. The main current policy along these lines is for men having unprotected anal sex with other men (MSM). Undoubtedly there are circumstances when it is appropriate for women, but usually not in the average sexually active women in a dating or hooking up environment. Instead I would advise against unprotected vaginal or anal sex -- i.e. to consistently use condoms -- as more effective protection. (Oral sex is low enough risk that condoms are optional.) After all, antibiotics are zero protection against the biggie, HIV infection; and also of course ineffective in preventing herpes, HPV, and other viral infections.

Second, and perhaps most pertinent to your question, if you go down the antibiotic path, azithromycin is a bad choice. Gonorrhea and syphilis are often resistant to it, and azithromycin would reduce your risk only about half; and if you acquired gonorrhea despite azithromycin, you would contribute to still further resistance of gonorrhea. And azithromycin is no longer the drug of choice for chlamydia:  it is only 80-90% effective as treatment in women; doxycycline for 7 days is now the standard recommended treatment.

If you proceed along these lines, consider post exposure doxycycline instead, in a dose of 200 mg within 72 hours of exposure. That's the recommendation for MSM following high risk anal sex. It is 100% effective in preventing chlamydia and nearly as good against syphilis, although only about 50% protective against gonorrhea.

I hope these thoughts help you make an intelligent, rational decision on this. Regardless of what you choose, please also be careful about your partners and get tested regularly, especially for gonorrhea, chlamydia, syphilis and HIV. Get vaccinated against HPV and hepatitis B, if not already done, and follow standard Pap smear recommendations.

Best wishes to you. Let me know if anything isn't clear.

HHH, MD

This being my last question- can you comment on the percentage of asymptomatic chlamydia in men? - I did not realize that throat chlamydia was not transmitted to penis, that is good information to know. Even if oral chlamydia self cures in a few weeks, would taking the azithromycin combat an infection if exposed? 

I hope these comments are helpful. As you apparently understood, that concludes this thread. Best wishes and say safe -- without unneeded antibiotics!