Check out the following link.
https://www.cdc.gov/std/tg2015/syphilis.htm
Penicillin G, administered parenterally, is the preferred drug for treating persons in all stages of syphilis. The preparation used (i.e., benzathine, aqueous procaine, or aqueous crystalline), dosage, and length of treatment depend on the stage and clinical manifestations of the disease. Treatment for late latent syphilis and tertiary syphilis require a longer duration of therapy, because organisms theoretically might be dividing more slowly (the validity of this rationale has not been assessed). Longer treatment duration is required for persons with latent syphilis of unknown duration to ensure that those who did not acquire syphilis within the preceding year are adequately treated.
Here's another interesting link.
https://aac.asm.org/content/61/1/e01092-16
I know guys who have successfully treated C, G and S after coming home with bugs from a cream filled Southern Decadence weekend or a vacation in Mexico with a cocktail of doxycycline, tetracycline, and azithromycin. You need a Dr. who knows you well enough to prescribe this type of treatment proactively or for when you need a travel pack.