Medical marijuana is legal in 23 states and the District of Columbia, so you’d think we have plenty of evidence about how it works for different health conditions. Does it really help with things like cancer pain, nausea, low appetite and glaucoma? Is it safe to use?

The truth is, there isn’t as much research on the plant’s medical effects as there should be. To be sure, there’s plenty of research on how pot affects adolescent brains, but that’s not about medical marijuana. There is surprisingly little known about how well marijuana, or its cannabinoid compounds, works for adults with specific medical conditions. The resulting confusion often extends to state regulations. In California, for example, marijuana is approved for the treatment of post-traumatic stress disorder (PTSD)…but in Colorado, it’s not.

To provide clearer answers, we reviewed a recent analysis of the existing medical literature in JAMA—and then spoke with an expert, David Casarett, MD, a palliative care doctor at University of Pennsylvania’s Perelman School of Medicine and author of the new book Stoned: A Doctor’s Case for Medical Marijuana. In spite of the seemingly one-sided title, Dr. Casarett wants us to know not just about the potential benefits of medical marijuana for certain conditions—but to understand its very real potential for dangerous side effects as well.

“Marijuana is definitely beneficial for some conditions, such as neuropathic pain and muscle spasms in MS,” says Dr. Casarett. “It offers much more benefit than I would have thought possible when I started the research for this book, so that’s one surprise. The other surprise was the risks—addiction, dependence, driving risks, other risks such as psychotic episodes, maybe an increased risk for heart attack and strokes.”

We asked Dr. Casarett for his clinical judgment about some of the specific medical conditions covered in the JAMA review. Why go beyond the statistical analysis? Because medicine is more than just research statistics. It’s about combining scientific evidence with clinical knowledge and patient experience. If you’re considering medical marijuana for a health condition, you’ll need to make up your own mind. We hope Dr. Casarett’s insights help.


In the JAMA article, British researchers reviewed the best-quality evidence they could find about medical marijuana as well as natural and synthetic cannabinoids. First, they looked for randomized clinical trials (RCTs), in which treatment with marijuana was compared with no treatment or a placebo. If there weren’t any such studies, they allowed in uncontrolled studies that had at least 25 subjects. That let them winnow down the research to 79 studies.

None were of the highest quality—long-term studies with a large set of subjects and extensive controls against bias. But many were of moderate quality—reasonably good evidence for effectiveness. Others were low quality, and some very low quality—basically not trustworthy.

One surprise was that there was only one randomized clinical trial on treating glaucoma, a common use for marijuana…and that study showed no benefit. Earlier preliminary studies had shown that marijuana reduces eye pressure for three or four hours at a time. “It used to be recommended, but now it’s not, mostly because there are other drugs that are more effective and longer-lasting,” says Dr. Casarett.

Studies on other conditions did show benefits.

There is moderate-quality evidence that marijuana helps…

• Chronic neuropathic pain

• Cancer pain

• Muscle spasticity due to multiple sclerosis.

There is low-quality evidence that marijuana helps…

• Nausea and vomiting due to chemotherapy

• Weight gain in HIV infection

• Sleep disorders

• Tourette syndrome.

There is very low-quality evidence that marijuana helps…

• Anxiety

• Depression.


Dr. Casarett agrees with most of the results of this analysis, although his interpretation of some of the research differs in some cases. He also brings in clinical experience that provides a different perspective for these conditions…

• Neuropathic pain. This is pain that’s caused by, among other things, nerve damage from diabetes, chemotherapy (and in some cases cancer itself), trauma and autoimmune diseases such as lupus. The positive evidence from moderate-quality studies is particularly welcome news for anyone experiencing this condition because it’s hard to treat…and because opioids, which are commonly prescribed for it, usually aren’t very effective. “Morphine and other opioids often don’t relieve the pain, but they do make people feel kind of sick and goofy,” says Dr. Casarett. Even if marijuana doesn’t replace opioids entirely, it can help patients cut back on their dosages—and that cuts not just side effects but the very real dangers of opioid overdose. “In states where they’ve legalized marijuana, rates of opioid overdose deaths have dropped,” he notes. “Many patients I’ve spoken with have used marijuana to get off their other drugs.”

• Muscle spasticity that comes with multiple sclerosis (MS). There is good evidence that this works. While MS is relatively rare, Dr. Casarett would like to see more research into marijuana as a treatment for other kinds of muscle spasms, such as those in low back pain. He doesn’t recommend it for that purpose at this time, however.

• Improving appetite if you have advanced cancer or AIDS. While marijuana hasn’t been shown to help people with these conditions gain weight, it has been shown to increase appetite, which is important to preserving the quality of life.

• Nausea, especially after chemotherapy. Here, Dr. Casarett disagrees with the British review, which rated the evidence as low. “I thought the evidence was pretty convincing,” he says. As a doctor, Dr. Casarett has a different perspective as well. “I may have a patient who says, ‘Maybe there have only been a few small studies, but I’ve had this nausea now for a week after chemotherapy…I’m miserable…I can’t keep anything down…I’ve tried four medications…I’m just going to die if I don’t get relief.” His conclusion: “For me there is enough evidence to recommend its use for nausea after chemotherapy.”

• Sleep disorders. While there are few studies on marijuana and sleep disorders, Dr. Casarett believes that marijuana can be helpful in certain situations—when your symptoms from other medical conditions are what’s really keeping you awake. In trials for MS and neuropathic pain, for example, patients who got marijuana slept better. But there isn’t enough evidence to recommend it for insomnia per se.

• PTSD and other conditions. Just because marijuana hasn’t been studied for a particular condition doesn’t mean it won’t work. We just don’t know. “I know a lot of people, for instance, who use it for PTSD,” he says. “Yet there is no randomized controlled trial data that shows that it works for PTSD.” In general, he says, “There is an enormous gap between what we’re pretty sure it works for and what people are using it for.” That’s why we need much more research.


If you’re considering using medical marijuana, you can learn about the different forms (smoking, vaping, edibles, tinctures) and other practicalities in our Bottom Line Guide to Marijuana: Its Health Benefits and Dangers. According to Dr. Casarett, we don’t know enough to say that particular forms of marijuana or extracts (natural or synthetic) work best for specific conditions.

Marijuana affects different people differently, so it’s best to proceed with caution. That’s especially true if you are older, are taking other medications, have experienced balance problems, have psychological issues or are concerned about cognitive function. You may experience dizziness, disorientation, drowsiness, confusion, loss of balance and even hallucinations. Dry mouth and nausea are also common side effects. “You can’t fatally overdose on marijuana, but it can still make you feel really goofy, panicked and paranoid,” Dr. Casarett says.

Many middle-aged and older adults start with edibles, which have their own risks. “If they tell you a dose is one chocolate bar, start out with one square rather than the whole bar. In general, I think it pays to be cautious and to underestimate the first couple of doses and gradually work your way up.” It’s also important to note that no one knows what the perfect dose of medical marijuana (of any form) is, so less than the recommended amount may be enough for you. Plus, a recent study found that cannabinoid concentrations in edibles often are mislabeled (about one-quarter of the products tested were stronger than their labels indicated).

Finally, if you’re not getting it legally, you should still consider telling your doctor that you’re using marijuana so he/she can advise you of the risks. Some people can become dependent or even addicted, he notes. Because marijuana can raise your heart rate (even doubling it), there’s a concern for people who are already at risk for heart attack or stroke. It’s also dangerous (and illegal) to drive while you’re under the influence of marijuana. “Hopefully, you have a physician with whom you’re comfortable enough to say, for example, ‘I’m using this for nausea, and I just wanted to let you know in case you have any advice for me.'”