LONDON — Cannabis-based medicines were approved on Monday for use by the National Health Service in England and Wales, a milestone decision that could change the lives of thousands of patients.

Three treatments using medicinal cannabis were authorized by the National Institute for Health and Care Excellence, a public body that provides guidance on health care practices. The decision comes a year after Sajid Javid, then the British home secretary, said that some doctors could legally prescribe the drug in special cases.

Cannabis-based medicine will be restricted to treatments for one of two relatively rare forms of childhood epilepsy, multiple sclerosis, or chemotherapy-induced vomiting and nausea, according to the guidelines published by the institute.

All marijuana-based medicines must initially be prescribed by specialist doctors, the institute said in a statement. A spokesman said in an email on Monday that the recommendations could immediately be applied for England and Wales, with Northern Ireland expected to consider use on a case-by-case basis. The rules do not apply in Scotland, which has its own health care guidance body.