The movement to legalise marijuana for medical use in the United States of America has renewed discussion about how this drug affects the brain, and whether it might be useful in treating psychiatric disorders.

Unfortunately, most of the research on marijuana is based on people who smoked the drug for recreational rather than medical purposes. A review by researchers in Canada (where medical marijuana is legal) identified only 31 studies (23 randomised controlled trials and eight observational studies) specifically focused on medical benefits of the drug.

A separate review by the American Medical Association (AMA) also concluded that the research base remains sparse. This was one reason that the AMA urged the federal government to reconsider its classification of marijuana as a Schedule 1 controlled substance (prohibiting both medical and recreational use), so that researchers could more easily conduct clinical trials.

Consensus exists that marijuana may be helpful in treating certain carefully defined medical conditions. In its comprehensive 1999 review, for example, the Institute of Medicine (IOM) concluded that marijuana may be modestly effective for pain relief (particularly nerve pain), appetite stimulation for people with AIDS wasting syndrome, and control of chemotherapy-related nausea and vomiting.

Given the availability of FDA-approved medications for these conditions, however, the IOM advised that marijuana be considered as a treatment only when patients do not get enough relief from currently available drugs. Additional research since then has confirmed the IOM’s core findings and recommendations.

Although anecdotal reports abound, few randomised controlled studies support the use of medical marijuana for psychiatric conditions. The meager evidence for benefits must be weighed against the much better documented risks, particularly for young people who use marijuana.

Key points

  • Medical marijuana may be an option for treating certain conditions, such as nerve pain or chemotherapy-related nausea.
  • There is not enough evidence to recommend medical marijuana as a treatment for any psychiatric disorder.
  • The psychiatric risks are well documented, and include addiction, anxiety, and psychosis.

Challenges in drug delivery

Marijuana is derived from the hemp plant, Cannabis. Although marijuana contains more than 400 chemicals, researchers best understand the actions of two: THC (delta-9-tetrahydrocannabinol) and cannabidiol.

THC is the chemical in marijuana primarily responsible for its effects on the central nervous system. It stimulates cannabinoid receptors in the brain, triggering other chemical reactions that underlie marijuana’s psychological and physical effects — both good and bad.

Less is known about cannabidiol, although the research suggests that it interacts with THC to produce sedation. It may independently have anti-inflammatory, neuroprotective, or antipsychotic effects, although the research is too preliminary to be applied clinically.

Drug delivery remains a major challenge for medical marijuana. The FDA has approved two pills containing synthetic THC. Dronabinol (Marinol) combines synthetic THC with sesame oil. Most of the active ingredient is metabolized during digestion, however, so that only 10% to 20% of the original dose reaches the bloodstream. Nabilone (Cesamet) uses a slightly different preparation of synthetic THC that is absorbed more completely into the bloodstream. Among the concerns about both of these drugs, however, are that they do not work rapidly, and the amount of medication that reaches the bloodstream varies from person to person.

Another medication under investigation in the United States (and already approved for sale in Canada) combines THC and cannabidiol. In Canada, it is marketed as Sativex. This drug is sometimes referred to as “liquid cannabis” because it is sprayed under the tongue or elsewhere in the mouth, using a small handheld device. However, it takes time to notice any effects, as the drug has to be absorbed through tissues lining the mouth before it can reach the bloodstream.

Inhalation is the fastest way to deliver THC to the bloodstream, which is why patients may prefer smoking an herbal preparation. But while this method of drug delivery works fast, smoking marijuana exposes the lungs to multiple chemicals and poses many of the same respiratory health risks as smoking cigarettes. Limited research suggests that vaporizers may reduce the amount of harmful chemicals delivered to the lungs during inhalation.

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