In the old soapmaking books from the mid 1800s through the early 1900s, there are often recipes for medicinal soaps. Some of the medicinal ingredients included mercury and arsenic (yikes!), zinc, salicylic acid, thymol, camphor, bromine, coal tar, pine tar, turpentine, sulfur, phenol (carbolic acid), peroxide, witch hazel, iodine, formaldehyde, tannin, etc.
Bear in mind that many of these medicinal soaps were used in the days when germ theory was unknown or in its infancy, and there were few if any antibiotics available. Formaldehyde, phenol, and iodine were harsh disinfectants that were the best option of the day. These chemicals and heavy metals like mercury deserve to be retired from common use now that we have safer, milder, and effective alternatives.
One author of the day noted:
"...Soap is often used for the conveyance of various medicants, antiseptics or other material presumably beneficial for treatment of skin diseases. While soap is an ideal medium for the carrying of such materials, it is an unfortunate condition that when incorporated with the soap, all but a very few of the numerous substances thus employed lose their medicinal properties and effectiveness for curing skin disorders, as well as any antiseptic value the substance may have.
"Soap is of such a nature chemically that many of the substances used for skin troubles are either entirely decomposed or altered to such an extent so as to impair their therapeutic value. Thus many of the claims made for various medicated soaps fall flat, and really have no more antiseptic or therapeutic merit than ordinary soap which in itself has certain germicidal and cleaning value.
"In medicating a soap, the material used for this purpose is usually added at the mill. A tallow and cocoanut oil base is best adapted for a soap of this type...."
From E G Thomssen, Soap-making Manual, 1922.
Another author cautioned:
"...Every soapmaker engaged in the preparation of medicated soaps is aware of the painstaking care and accuracy required to make soaps according to a physician's prescription and, although many manufacturers turn out an excellent product, there are many so-called medicated soaps brought into commerce which do not deserve the name, since they frequently contain not even a trace of efficacious substances.
"Again, we find other medicated soaps which are inoperative on account of ignorance as regards the decomposition of the chemical agents added to the soap. In many iodine soaps, for instance, a small addition of potassium iodide is found, biit no sodium hyposulphite, which is absolutely necessary to render the iodine eflBcacious.
"And, thus there are many other soaps, which from a want of sufficient chemical knowledge are not properly composed, and, instead of having a soothing and healing effect in skin diseases, promote the diseased state...."
From C Diete, The Soap Maker's Handbook..., 1912.