What happens if you can't have a penicillin shot for syphilis?

Doxycycline: Doxycycline for 2-4 weeks is recommended if penicillin cannot be given, or sometimes ceftriaxone. However, penicillin is by far the best drug. If you had a severe penicillin allergy in the past, then probably penicillin should be avoided. Otherwise, it still might be the best choice for you. Discuss it with your doctor or, even better, an infectious diseases or STD specialist.