DEAR DR. ROACH: What advice do you have for both doctors and patients for approaching medical marijuana use? What is your ideal vision of its future? — C.C.

ANSWER: I look at marijuana as a potentially useful medicinal plant that has several potential downsides. One of the downsides is an unrealistic expectation of its usefulness by believers. I increasingly see claims that marijuana (or one of its extracts) can treat or cure almost every known disease. It also is hampered by its status as a Schedule I drug by the Drug Enforcement Agency, so high-quality research is difficult to do. Finally, it has been used for many years as a recreational drug (which is partially responsible for the first two downsides). This has led to resistance to the idea among nonbelievers that it might have legitimate use. The psychological effects for which it is used recreationally can cause it to be intolerable for some. Further, a small percentage of people (estimates range between 2 percent and 6 percent) have difficulty quitting the drug, even when they want to.

Marijuana contains many potentially active chemicals, of which only two (THC and cannabidiol) have been studied well. The best evidence of effectiveness for any type of marijuana product is in epilepsy and chronic pain, but the quality of the data is limited. There are very few situations where I would consider prescribing medical marijuana, but they do exist.

Most of the risks of medical marijuana are also unknown. Anecdotal reports suggest that fewer users of medical marijuana are smoking it, in favor of ingesting extracts. I suspect that this will lead to lower risk of heart and lung problems, but I withhold judgment until there are good data. It also is clear that marijuana can cause or worsen anxiety in some people. Adolescents who use marijuana may be at higher risk of schizophrenia.

My ideal future regarding medical marijuana is one where it is studied openly and subjected to the same scrutiny as other medicines, where both the purified extracts are studied as well as the whole plant. Marijuana needs to be compared against the best treatments we have, not just against placebo. Only this way can we confirm or refute the benefits and risks of this drug.

DEAR DR. ROACH: My physician has recommended a colonoscopy. I’ve never had one and am overdue. My question involves the prep protocol. My wife had one with the doctor who will do mine at the local hospital, and he prescribed a liquid prep mixture that my wife found quite unpleasant. A friend from out of state who had a colonoscopy was given a single pill as a prep. Please discuss the difference and efficacy of these two types of prep. I’m inclined to ask the doctor doing my colonoscopy for the pill prep rather than the liquid. — B.W.S.

ANSWER: There are several different preparations available. Most of them involve a fairly large amount of liquid: 2 to 4 liters. I’m afraid my experience is like your wife’s: The preparations are not particularly pleasant, but drinking them quickly, having them cold and using flavorings in them (such as Mio or Crystal Light, but not red-colored ones, though) can make them much more tolerable.

Pill-based colonoscopy preparations, like your friend had, are based on sodium phosphate, and they are used less frequently now because of safety concerns. This is especially true in people whose kidneys are less than perfect (which includes a lot of older people). I recommend the lower-volume liquid (polyethylene) preparation as a good balance between safety and tolerability.

* * *

Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers may email questions to ToYourGoodHealth@med.cornell.edu or request an order form of available health newsletters at 628 Virginia Dr., Orlando, FL 32803. Health newsletters may be ordered from www.rbmamall.com.