The term 'medicinal cannabis' is used to describe a product derived from cannabis which is prescribed by a health professional to treat a medical condition. Since 2018 it has been legal in the UK for specialist doctors to prescribe cannabis based medicinal products for a small number of medical conditions, e.g. certain childhood epilepsies.
Cannabis based medicinal products can vary considerably. Some are produced by pharmaceutical companies according to rigorous production methods to ensure standardised strength of contents such as THC and CBD. Other products are produced to less rigorous standards with contents that may vary between batches.
The evidence for the benefits and risks of cannabis based medicines are still developing, as is the understanding of which medical conditions they treat. As this evidence builds, the guidelines for use of cannabis based medicines are likely to develop.
It's illegal to use medicinal cannabis without it being prescribed to you by a doctor.