In the late 1400s and early 1500s mercury was used to treat the disease. It was rubbed on the skin, administered by mouth and even injected into the patient! That is where the saying "A night in the arms of Venus leads to a lifetime on Mercury" originates from.
Another method was fumigation. The patient was placed in a box with his head sticking outward. Mercury was then placed inside the box.
A fire was lit underneath and that would cause the mercury to vaporize. It was a very unpleasant process for the patient and not very effective at all.
In the 1900's it was noticed that syphilis was cured by high fevers. There was a disease that could cause very high fevers indeed – malaria.
Syphilis patients were inoculated with malaria in the hopes that it would induce a high fever and cure the patient from syphilis. It was a fair gamble as the malaria could later be treated with quinine, which was already available.
Sweat-boxes were later used and these methods were only rendered unnecessary by the discovery of penicillin and its widespread manufacture after World War II. Penicillin is a reliable and effective treatment method for syphilis.
