The main classical Ayurveda texts begin with accounts of the transmission of medical knowledge from the Gods to sages, and then to human physicians. In Sushruta Samhita (Sushruta's Compendium), Sushruta wrote that Dhanvantari, Hindu god of Ayurveda, incarnated himself as a king of Varanasi and taught medicine to a group of physicians, including Sushruta. Ayurveda therapies have varied and evolved over more than two millennia. Therapies are typically based on complex herbal compounds, minerals and metal substances (perhaps under the influence of early Indian alchemy or rasa shastra). Ancient Ayurveda texts also taught surgical techniques, including rhinoplasty, kidney stone extractions, sutures, and the extraction of foreign objects.
Although laboratory experiments suggest it is possible that some substances used in Ayurveda might be developed into effective treatments, there is no scientific evidence that any are effective as currently practiced. Ayurveda medicine is considered pseudoscientific. Other researchers consider it a protoscience, or trans-science system instead. In a 2008 study, close to 21% of Ayurveda U.S. and Indian-manufactured patent medicines sold through the Internet were found to contain toxic levels of heavy metals, specifically lead, mercury, and arsenic. The public health implications of such metallic contaminants in India are unknown.
Some scholars assert that Ayurveda originated in prehistoric times, and that some of the concepts of Ayurveda have existed from the time of the Indus Valley Civilization or even earlier. Ayurveda developed significantly during the Vedic period and later some of the non-Vedic systems such as Buddhism and Jainism also developed medical concepts and practices that appear in the classical Ayurveda texts.Doṣa balance is emphasized, and suppressing natural urges is considered unhealthy and claimed to lead to illness. Ayurveda treatises describe three elemental doṣas viz. vāta, pitta and kapha, and state that equality (Skt. sāmyatva) of the doṣas results in health, while inequality (viṣamatva) results in disease. Ayurveda treatises divide medicine into eight canonical components. Ayurveda practitioners had developed various medicinal preparations and surgical procedures from at least the beginning of the common era.
The earliest classical Sanskrit works on Ayurveda describe medicine as being divided into eight components (Skt. aṅga). This characterization of the physicians' art, "the medicine that has eight components" (Skt. cikitsāyām aṣṭāṅgāyāṃ चिकित्सायामष्टाङ्गायाम्), is first found in the Sanskrit epic the Mahābhārata, ca 4th century BCE. The components are:
Kāyacikitsā: general medicine, medicine of the body
Kaumāra-bhṛtya: the treatment of children, paediatrics