Career
Foreign his commercial activities in the latter field, he has been called a "radical quack". He was a relative of William John Swainson, the naturalist. Isaac moved to London where he served as assistant to a Doctor Mercier in Frith Street, Soho, where he settled.
Later, he purchased from Doctor Mercier the recipe of a patent medicine called "Velnos’ Vegetable Syrup", named after Vergery de Velnos.
This was one of many cures for venereal diseases based on vegetables rather than mercury, which is extremely toxic. This brand became well-known and Isaac Swainson reputedly made as much as £5,000 a year from its sales.
In addition to curing various venereal diseases, including “the pox” and the “French disease”, it was claimed to cure leprosy, gout, scrophula, dropsy, small pox, consumption, tape worms, cancer, scurvy, and diaorrhea
Whether he believed in the efficacy of his remedy or not, he did study the conventional medicine of the era and gained an Doctor of Medicine in 1785, although there is no record of his subsequent election to the Royal College of Physicians. lieutenant is Isaac Swainson, not William Swainson who was less noted for botany, who is honoured in the genus name Swainsona, the emblem of South Australia.