Ayurvedic medicine is a system of traditional medicine native to the Indian subcontinent and a form of alternative medicine. The oldest known Ayurvedic texts are the Suśrutha Saṃhitā and the Charaka Saṃhitā. These Classical Sanskrit texts are among the foundational and formally compiled works of Ayurveda. By the medieval period, Ayurvedic practitioners developed a number of medicinal preparations and surgical procedures for the treatment of various ailments. Practices that are derived from Ayurvedic medicine are regarded as part of complementary and alternative medicine, and along with Siddha Medicine and Traditional Chinese medicine, forms the basis for systems medicine. Concerns have been raised about Ayurvedic products; U.S. studies showed that up to 20% of Ayurvedic U.S. and Indian-manufactured patent medicines sold through internet contained toxic levels of heavy metals such as lead, mercury and arsenic.