Doxycycline will affect and also cure syphilis, if taken early enough. It's hard to answer your question. If you're not having any sex with anyone meanwhile, then it doesn't hurt to take the Doxy and get cured of syphilis. However, if you had sex with someone, and then took the Doxy, then you'd never know if you had syphilis and infected your sexual partner. However, syphilis is not a very common STD. What was your exposure?