It’s now prescribed for nerve pain, nausea, certain types of cancer and more.

Would you use cannabis (marijuana) if your doctor recommended it for, say, nerve pain or to help control muscle spasms? Twenty-three states, plus the District of Columbia, have now legalized it for such medical uses.*

But do the benefits of medical marijuana truly outweigh the risks? Who should consider trying it—and what’s the best way to use it safely? To learn more, Bottom Line/Health spoke with cannabis expert Gregory T. Carter, MD…

Why has marijuana suddenly become so popular as a medical treatment? There’s really nothing new about the medical use of cannabis. It was included in the US Pharmacopeia (the official authority for prescription and over-the-counter drugs) until the early 1940s, when, due largely to political opposition, it was removed. For this reason, it has been difficult to study cannabis for medical purposes.

The US government still classifies cannabis as a Schedule 1 drug—that is, a substance with no medical benefits and a high potential for abuse. Although federal law prohibits the use of medical marijuana, doctors now prescribe it in states where it has been legalized by the state legislatures.

What conditions is marijuana most likely to help? Several—but because research has been impeded by political considerations, the level of evidence varies for each condition.

Beginning nearly 30 years ago, the prescription drug dronabinol (Marinol) was approved by the FDA for treating appetite loss in AIDS patients. It’s a synthetic version of one of the active compounds in cannabis.

Since then, cannabis has been shown to relieve nausea in cancer patients. Unlike the synthetic version, it’s not a single agent. It contains 80 to 100 different medicinally active compounds known as cannabinoids.

New lab and animal studies suggest that cannabis can be an effective treatment for cancer itself. The active compounds have been shown to limit tumor invasiveness and the activation of chemical signals that stimulate growth in some types of cancers.

Patients who have nerve pain often get more relief with cannabis than with other prescription medications, such as gabapentin (Neurontin). Cannabis also has been used successfully to treat muscle spasms, seizures and other neurological conditions such as multiple sclerosis. And it seems to be effective at controlling the tics and behavior problems caused by diseases such as Tourette’s syndrome. Some people also use it for fibromyalgia and glaucoma.

If my doctor prescribes cannabis, what type should I buy? Cannabis dispensaries (in states where medical uses are permitted) typically sell many different strains, which affect users in different ways. The best medical strains have a high concentration of cannabidiol (CBD) and cannabinol (CBN), with relatively small amounts of tetrahydrocannabinol (THC), the compound responsible for most of the intoxicating effects.

Bear in mind that even in states that have legalized it, insurance does not cover the cost of medical marijuana, which ranges from $200 to $500 an ounce.

Is it better to smoke medical marijuana or eat it? Neither. It’s unlikely that the small amount of smoke from medical doses increases the risk for emphysema or other lung diseases, but the smoke does contain carcinogens. Why take chances? Edible forms of cannabis—such as baked goods, shakes and candies—are effective, but it’s difficult to control your intake. The absorption rate will depend on what’s already in your stomach, and it can take up to two hours to feel the effects.

The best way is to use a vaporizer. The medicinal compounds vaporize at a much lower temperature than is required for combustion (smoking). The inhaled vapor gives the same rapid onset as smoking—you’ll feel the effects within a few minutes. Vaporized cannabis is safer than inhaled smoke. Vaporizers are sold online and at dispensaries for around $100 to as much as $400.

You can also buy sublingual tinctures at dispensaries. You put a few drops under your tongue. You’ll feel the effects almost as quickly as from smoking. This form of medical marijuana can be just as effective as other forms, but concentrations are not standardized, so it can be a bit harder to provide the correct dose.

What is the right dose? Everyone reacts differently to cannabis, and some strains are more potent than others. If your doctor prescribes cannabis, don’t leave the office without detailed instructions—how much to use…how often…and what to expect.

Marijuana also interacts with certain medications you may be taking, such as sedatives, including the tranquilizer lorazepam (Ativan) and the sleep drug zolpidem (Ambien), so be sure to also discuss all medications you take.

What are the risks? Your motor skills and reaction times will be diminished, particularly within the first few hours. You don’t want to drive, operate power tools or engage in other tasks that require serious concentration while the drug is active in your body, which varies widely depending on the person and the strain of cannabis but is typically four to six hours.

What about addiction? Few people who use cannabis for medical purposes will become dependent, develop cravings or go on to abuse other drugs. But the potential for addiction is obviously a concern, just as it is with alcohol and some types of medication. If you have a history of alcoholism or other forms of addiction, talk to your doctor about this before using ­cannabis.

* Medical marijuana has been legalized in Alaska, Arizona, California, Colorado, Connecticut, District of Columbia, Delaware, Hawaii, Illinois, Maine, Maryland, Massachusetts, Michigan, Minnesota, Montana, Nevada, New Hampshire, New Jersey, New Mexico, New York, Oregon, Rhode Island, Vermont and Washington. Seven states—Alabama, Florida, Iowa, Mississippi, Tennessee, Utah and Wisconsin—allow medical marijuana for specific conditions, such as epilepsy.