The quality demands of the pharmaceutical industry require prescription medicines to be consistent in their active ingredient content. Achieving this, using raw cannabis as a feedstock, is especially challenging. The plant material is extremely inhomogeneous, and the ratios of active ingredients are affected by a range of factors. These include the genetics of the plant, the growing and storage conditions, the state of maturity at harvest, and the methods used to process and formulate the material. The reasons for this variability are described, with particular emphasis on the botanical considerations. To produce the complex botanical medicine Sativex®, which contains the cannabinoids Δ9–tetrahydrocannabinol (THC) and cannabidiol (CBD) and a range of other ingredients, GW Pharmaceuticals had to manage these variables. This medicine, for the treatment of spasticity due to multiple sclerosis, is the first cannabis‐based medicine to be approved in the UK. The company's methodology for producing this and other chemotypes is described. Copyright © 2013 John Wiley & Sons, Ltd.