That's right - they helped reduce the chancres or sores.
Of course they were toxic in their own right.
Mercury was considered a state of the art cure for health problems in the early nineteenth century. The Lewis and Clark expedition took along a large supply.
This fact allowed modern archaeologists to precisely map out the expedition's route. They just went to potential sites and tested the soil. Whenever they found mercury contamination they knew they had located a latrine.
It is sometimes suspected that Lewis killed himself in the throes of syphilitic dementia. Or perhaps it was dementia caused by mercury poisoning. Or dementia caused by malaria. Or dementia caused by opium. Or it's possible that he was murdered.
Stories like Lewis's make me feel better about modern medicine. Although of course modern medicine has its own horror stories.
There is a book, “Heavenly Intrigue” that argues that Tycho likely died of mercury poisoning administered by Kepler. Tycho, besides running an observatory in Prague, had a medical shop, where he formulated mercury compounds to treat a disease he contracted while a student, around the same time he lost his nose in a sword fight. (He was a nobleman student and had a chaperon, who was probably as drunk and whore-infested as he was.)
The formulary produced various mercury compounds in the process, some of which were known to be toxic. Kepler certainly had means, motive and opportunity. No one benefited from Tycho’s very opportune death as much as Kepler, for whom it was a godsend, both professionally and financially.