Siddha medicine is a form of traditional medicine originating in southern India. It is one of the oldest systems of medicine in India. The Indian Medical Association regards Siddha medicine degrees as "fake" and Siddha therapies as quackery, posing a danger to national health due to absence of training in science-based medicine. Identifying fake medical practitioners without qualifications, the Supreme Court of India stated in 2018 that "unqualified, untrained quacks are posing a great risk to the entire society and playing with the lives of people without having the requisite training and education in the science from approved institutions".

In rural India, siddhars have learned methods traditionally through master-disciple relationships to become local "healers" known as siddhars. Siddhars are among an estimated 400,000 traditional healers practicing medicine in India, comprising some 57% of rural medical care. Siddha practitioners believe that five basic elements – earth, water, fire, air, sky – are in food, "humours" of the human body, and herbal, animal or inorganic chemical compounds, such as sulfur and mercury, used as therapies for treating diseases.

The Ministry of Ayurveda, Yoga and Naturopathy, Unani, Siddha and Homoeopathy of the Government of India regulates training in Siddha medicine and other traditional practices grouped collectively as AYUSH. The Tamil Nadu Dr. M.G.R Medical University offers courses with advanced degrees, such as BSMS (Bachelor in Siddha Medicine and Surgery), MD (Medical Doctor, Siddha) or Doctor of Philosophy (PhD). The Central Council of Indian Medicine, a statutory body established in 1971 under AYUSH, monitors education in areas of Indian traditional medicine, including Siddha medicine. Siddha degree holders can become registered Siddha practitioners and are allowed to prescribe drugs as per the standards recorded in the Siddha Pharmacopoeia of India (SPI) under the Drugs & Cosmetics Act, 1940. However, modern medicine prescriptions by Siddha practitioners are also considered as quackery by the Indian Medical Association.

Siddha is an ancient Indian traditional treatment system which evolved in South India, chiefly in Tamil Nadu. According to ancient literature of Siddha, it is said that the system of this medicine originated from Hindu God Shiva who taught it to his consort Parvati. Parvati then passed it on to Nandi and Nandi taught about it to nine Devtas.

Though the origin of this system is considered to be divine, Siddhar Agasthyar is considered as the founding father of this medical system. There are 18 prominent siddhars who are the main contributors to this system of medicine. The original texts and treatise for siddha are written in Tamil language.

Siddha views disease as a condition caused when the normal equilibrium of the three humors (collectively called mukkuttram) – vaadham (airy), pittham (fiery) and kapam (watery) – is disturbed. The factors assumed to affect this equilibrium are environment, climatic conditions, diet, physical activities, and stress. Under normal conditions, the ratio between Vaadham, Pittham, and Kapam are 4:2:1, respectively.

According to the Siddha medicine system, diet and lifestyle play a major role in health and in curing diseases. This concept of the Siddha medicine is termed as pathiyam and apathiyam, which is essentially a rule-based system with a list of "do's and don'ts".

The herbal agents used by the siddhars could be classified into three groups: thavaram (herbal product), thadhu (inorganic substances) and jangamam (animal products). The thadhu agents are further classified as: uppu (water-soluble inorganic substances that give out vapour when put into fire), pashanam (agents not dissolved in water but emit vapour when fired), uparasam (similar to pashanam but differ in action), loham (not dissolved in water but melt when fired), rasam (substances which are soft), and ghandhagam (substances which are insoluble in water, like sulphur).