Siddhars had an unparallel knowledge of medicine. They had the knowledge of processes like the calcination of the mercury minerals and metals and transmuting them for use as medicines even in the remote past when chemical technology was not fully developed. 

Siddhas are said to have supernatural powers. They were capable of performing alchemy-the act of converting base metal into gold. Their ability includes the art of making dead persons enter the body of the living for a few minutes, to give expression to certain ideas.  Being men of vision who know the past, the present and the future, the Siddhas are supposed to be capable of living and dying at will.  They are reputed to have used certain herbal leaves to intoxicate themselves and get into a trance or hypnotic state.

It is on record that the Siddhas who lived in Kanjamalai converted base metal into gold and experts certified that the gold made by the Siddhas with their miraculous powers contained seven parts of Gold as against ten parts of gold recovered in gold mines.

Certain Siddhas have powers even to nullify the fatal effects of snake poison. 

The use of 'Yantras' for curing diseases is widespread. These are certain mystic magical diagrams engraved on the arm or with the chain around the neck. A special prayer is held to instill divine powers into it. The Yantra is then assumed to be able to protect the worshippers from evil and to bestow on him health and wealth.

The first two of the 18 Siddhas, Nandi Thevar and Agasthya are said to have found out the medicinal value of herbs by a process of trial and error. And when the herbs turned out to cure diseases, the Siddhas planted the seedlings of the herbs in high altitudes and other inaccessible places. Bogar was another famous Siddhar who used chemicals for treating diseases.

Folklore belief is that there is no disease which cannot be cured by them. They are credited with the ability to confer longevity and eternal youth.