In August, researchers announced they had genetically engineered yeast to produce the powerful painkiller hydrocodone. Now comes the perhaps inevitable sequel: Scientists have created yeasts that can make important constituents of marijuana, including the main psychoactive compound, tetrahydrocannabinol, or THC.

Synthetic versions of THC are available in pill form under brand names like Marinol and Cesamet; they are generally used to treat nausea, vomiting and loss of appetite caused by HIV infection or cancer chemotherapy.

Genetically modified yeast could make THC in a cheaper and more streamlined way than traditional chemical synthesis.

Using yeast could also shed light on the clinical usefulness of cannabis-derived compounds. Marijuana is increasingly embraced as medicine, yet there is limited evidence that it is effective against many of the conditions for which it is prescribed.

Researchers hoping to separate fact from wishful thinking will need much better access to marijuana’s unique constituents. Modified yeast may...