Dear Dr. Roach: Recently in your column, you discussed the need for dosages to be exact in some situations. What about the medical marijuana issue? In our small town, there are four dispensaries. If someone truly felt he or she needed it for medical issues, would that person get the same dosage at each location? If the doctor prescribes aspirin, he doesn’t have me go to the willow tree, or to the willow dispensary down the street. — J.O.Answer: You are exactly right that the content of the active components of marijuana varies from strain to strain and even from plant to plant. This makes getting exact dosages impossible. This is a general problem with natural products, which is why Western medicine has preferred to identify, extract and purify the active ingredients.

There is a potential downside to this philosophy, which is that the purification may remove other substances, which may themselves have an effect or may modify the effect of a substance found in the original natural product. This appears to be the case with marijuana, as there are at least two compounds with important potential medical benefits, tetrahydrocannabinol (THC, which has several subtypes, especially delta-9 and delta-4) and cannabidiol (CBD). The effect of dronabinol (Marinol), a synthetic form of delta-9 THC, is reported as being very different from natural marijuana by most people who have used both, though this may be an effect of dose, of speed of onset, or of expectations. Hence the interest in medical marijuana, with its multiple compounds and ability for growers to emphasize the THC or CBD content.

Recreational users of marijuana have experience in achieving the correct dosage; however, for medical use (such as seizures), that ability isn’t relevant. I suspect the future will include a greater