In glasshouses dotted around England, more than a quarter of a million cannabis plants are growing under lights. They’ll soon be harvested and turned into Epidiolex, the first cannabinoid-based drug approved by the Food and Drug Administration for sale in the U.S. Botanical specialists have spent two decades cultivating the crop and refining the growing process—selecting plants just right for creating the oral medicine to combat certain types of severe epilepsy. A bio-pharmaceutical firm with sites in the English counties of Kent and Norfolk, GW Pharmaceuticals Plc is quick to distance themselves from the medical-marijuana industry. They say they don’t produce homeopathic elixirs for sale in herbal stores. The U.K. is the biggest producer of cannabis for medical and scientific purposes, according to the United Nations. Approved in June, the medicine could go on sale as soon as Thursday.