I never heard about doxycycline to prevent STD’s. Are you saying you can take it before you have sex & how often do you need to take it?
Doxycycline: Uses, Dosage, Side Effects, Warnings - Drugs.com
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Doxycycline is a tetracycline antibiotic used to treat many different bacterial infections such as acne, urinary tract infections, intestinal infections, and others.