Doxycycline 100 mg twice daily for 14 days is 100% effective against syphilis under a year in duration, assuming certain complications (such as neurosyphilis) are not present -- in which case 4 weeks treatment is advised. The treatment of choice remains a single injection of long-acting penicillin (benzathine penicillin G [BPG], trade name Bicillin LA®). However, there is a national shortage of BPG, and therefore doxycycline for 14 days is advised by CDC for those persons with early syphilis who are likely to remember to take all doses on schedule.

Tetracycline probably would be as effective but is not recommended, primarily because it must be taken 4 times daily and also more likely to cause stomach upset, therefore more difficult for some persons to take successfully.

Let me know if anything isn't clear.

HHH, MD

If you were treated with doxy to prevent syphilis -- i.e. after exposure but before primary syphilis developed -- 10 days is plenty.