If u are allergic to inj penicillin than Tetracycline, erythromycin, and ceftriaxone have shown antitreponemal activity in clinical trials.A 10- to 14-day trial of ceftriaxone is effective for treating early syphilis, although the optimal dose and duration have not been established. Doxycycline and tetracycline for 28 days have been used for many years and are the only acceptable alternatives to penicillin for the treatment of latent syphilis. Doxycycline is the preferred alternative to penicillin owing to its tolerability.