Can taking doxycycline be effective for post exposure prevention treatment after 24 hours for syphilis?

If you've had a known syphilis exposure, a better approach is a shot of benzathine (long acting) penicillin. If you rely on doxycycline, you need to take it twice daily for two weeks. If a high risk exposure with a partner not known to have syphilis, probably only a few doses of doxycycline are necessary, but the exact amount isn't really known.