Medical Marijuana: Benefits and Side Effects

Medically Reviewed by Jabeen Begum, MD on January 09, 2024
9 min read

Medical marijuana, or medical cannabis, uses the cannabis plant or chemicals in it to treat symptoms or conditions. Medical marijuana mostly comes in the same forms as recreational marijuana, but there also are highly purified and lab-made versions used for certain conditions.

The cannabis plant contains more than 100 different chemicals called cannabinoids. Each one has a different effect on the body. Delta-9-tetrahydrocannabinol (THC) and cannabidiol (CBD) are the main chemicals used in medicine. THC also produces the “high” people feel when they smoke marijuana or eat foods containing it.

As of 2023, a broad range of medical marijuana products were legal in 38 states, three territories, and the District of Columbia but remained illegal under federal law.

States where medical marijuana is legal have approved it for a varying list of conditions. Depending on where you live, these might include:

  • Severe and chronic pain
  • Multiple sclerosis and muscle spasms
  • Severe nausea and vomiting caused by cancer treatment
  • Epilepsy and seizures
  • Alzheimer's disease
  • Amyotrophic lateral sclerosis (ALS)
  • Crohn's disease
  • Glaucoma
  • Migraine
  • Anorexia
  • Extreme weight loss and weakness (wasting syndrome)
  • Irritable bowel syndrome
  • Post-traumatic stress disorder

It is important to know that many of the uses that states allow aren't backed by strong scientific evidence. This is partly because researchers find it hard to do studies on a drug that remains illegal under federal law, even when it's allowed by states.

It's also important to realize that cannabis is almost always used to treat symptoms -- like the muscle spasms of multiple sclerosis and the loss of appetite caused by HIV drugs -- rather than the conditions themselves.




Cannabinoids, the active chemicals in medical marijuana, are similar to chemicals the body makes that are involved in appetite, memory, movement, and pain.

So far, solid studies suggest that these chemicals can:

  • Control vomiting in people undergoing cancer chemotherapy. Drugs based on two lab-made forms of THC are FDA-approved for this purpose.
  • Modestly reduce pain in people with conditions such as nerve damage, cancer, multiple sclerosis, and rheumatoid arthritis.
  • Reduce muscle spasms in people with multiple sclerosis. (There's less evidence it helps muscle spasms in people with spinal cord injuries).

In addition, there's some evidence that medical marijuana might:

  • Improve sleep in the short term for people with obstructive sleep apnea, fibromyalgia, chronic pain, or multiple sclerosis.
  • Increase appetite and decrease weight loss in people with HIV and AIDS.
  • Reduce tics in people with Tourette's syndrome.
  • Reduce anxiety in people with social anxiety disorder.
  • Improve symptoms of post-traumatic stress disorder.

There's not enough evidence to say whether medical marijuana works or not for many other conditions, including irritable bowel syndrome, Parkinson's disease, and addiction to other substances.

In some cases, the limited evidence available suggests it doesn't help. That's the case with Alzheimer's and other forms of dementia. That's also the case with glaucoma, an eye disease that can be caused by increased pressure in the eyes. Marijuana might lower the pressure briefly, but not as long as effective drugs do.

Can medical marijuana help with seizure disorders?

Medical marijuana received a lot of attention a few years ago when parents said that a special form of the drug helped control seizures in their children. Studies showed good enough results for the FDA to approve Epidiolex, made from a purified form of CBD, as a therapy for people with two rare seizure disorders, Lenox-Gastaut and Dravet syndromes.

There's no approved form of medical marijuana for people with more common forms of epilepsy. Studies looking at broader uses are underway.

Medical marijuana can cause harm. The most serious harms include an increased risk of car crashes in intoxicated users and the risk that children might accidentally ingest these products and have breathing problems and other overdose symptoms. Adults over age 65 may also face an increased risk of injury when using marijuana.

Other short-term effects, including some that people enjoy and others that cause distress, can include:

  • Euphoria, which means a strong sense of happiness or well-being
  • Altered senses, such as seeing brighter colors or smelling stronger odors
  • An altered sense of space and time
  • Impaired memory and thinking
  • Confusion and disorientation
  • Poor muscle coordination
  • Relaxation or sleepiness
  • Dizziness
  • Panic attacks 
  • Dry mouth
  • Increased heart rate
  • Increased appetite
  • Slower reaction times
  • Harmful interactions with other drugs
  • Hallucinations (seeing things that aren't there) or delusions (believing things that aren't true). These tend to occur only at high doses.

Marijuana also may have long-term effects, including:

  • Negative effects on attention, learning, and memory in teens and young adults who use a lot of marijuana before their brains are fully developed.
  • Harms linked to smoking. Any kind of smoking can damage your lungs and increase your risk of stroke, heart disease, and blood vessel disease.
  • An increased risk of schizophrenia in people already at risk for this serious mental health disorder.
  • Frequent, severe vomiting, among longtime users of high-dose THC.
  • Addiction (cannabis use disorder). This is more likely to happen among teens than among adult users.

People who should not use marijuana include:

  • Those who are pregnant. THC can get into the fetal brain and affect later attention, memory, and problem-solving. Marijuana also can increase the risk of stillbirth, premature birth, and low birth weight.
  • Those who are breastfeeding. THC can also get into infant brains and may harm development.

Researchers say marijuana could also be dangerous for:

  • People with a history of psychosis, whose condition may get worse.
  • People with heart disease. Smoked marijuana, in particular, has clear heart risks. Studies find an increased risk of heart attack in the hour after smoking marijuana.


There are other safety issues to consider. The National Institute on Drug Abuse says marijuana can be addictive and is considered a “gateway drug” to using other drugs. But many other experts disagree. In fact, the Centers for Disease Control (CDC) states that most people who use medical marijuana do not go on to use “harder drugs” or street drugs. 

Also, the FDA doesn't oversee medical marijuana like it does prescription drugs, except purified and lab-made versions approved for certain uses. Although states monitor and regulate sales, they often don’t have the resources to do so effectively. That means the strengths and ingredients of marijuana can differ quite a bit depending on where you buy it. Products also may contain levels of active ingredients that differ from what's on their labels. Contaminated products, with high levels of pesticides or germs, also have been reported.

To get most forms of medical marijuana, you need a written recommendation from a licensed doctor in a state where that is legal. Note: this is not the same as a prescription, as these drugs (with just a few exceptions) are not FDA-approved prescription medicines. Not every doctor is willing to recommend medical marijuana for their patients. Some health care systems forbid the practice.

You must have a condition that qualifies for medical marijuana use in your state. Your state may also require you to get a medical marijuana ID card. Once you have that card, you usually can buy medical marijuana at a store called a dispensary. A few states also allow users to grow their own supply.

To take medical marijuana, you can:

  • Smoke it
  • Vape it (inhale it through a device that turns it into a mist)
  • Eat it, in forms such as a brownie, lollipop, or soft candy
  • Take it as a pill
  • Apply it to your skin using a lotion, spray, oil, or cream
  • Place a few drops of a liquid under your tongue
  • Insert a suppository in your vagina or anus

You should discuss these methods with your doctor because they work differently in your body and have different pros and cons for different people.

Smoking and vaping produce the quickest effects. They also can cause lung damage. The FDA has warned people not to vape marijuana products for that reason.

Cannabis products you eat -- edibles -- spare your lungs but have their own risks. They take longer to work, which leads some people to underestimate their potency and take too much. That can result in bad side effects, such as extreme sleepiness, anxiety, hallucinations, and vomiting.

There's less research on less common methods, such as suppositories.

More than two-thirds of U.S. states and the District of Columbia have legalized medical marijuana, in various forms, for medical treatments. More are considering bills to do the same. In some of those states, it's also legal for adults to use marijuana recreationally.

Medical marijuana laws differ from state to state, with some, for example, limiting THC doses or forbidding smoked forms.

Medical marijuana is legal in 38 states and the District of Columbia:

  • Alaska
  • Alabama
  • Arizona
  • Arkansas
  • California
  • Colorado
  • Connecticut
  • Delaware
  • District of Columbia
  • Florida
  • Hawaii
  • Illinois
  • Kansas
  • Kentucky
  • Louisiana
  • Maine
  • Maryland
  • Massachusetts
  • Michigan
  • Minnesota
  • Mississippi
  • Missouri
  • Montana
  • Nevada
  • New Hampshire
  • New Jersey
  • New Mexico
  • New York
  • North Dakota
  • Ohio
  • Oklahoma
  • Oregon
  • Pennsylvania
  • Rhode Island
  • South Dakota
  • Utah
  • Vermont
  • Virginia
  • Washington
  • West Virginia

Some states don't allow medical use of potentially intoxicating products high in THC. However, they do allow some access or provide legal protections for people using products low in THC but high in CBD for medical or research reasons. These states include Georgia, Indiana, Iowa, Kansas, North Carolina, South Carolina, Tennessee, Texas, Wisconsin, and Wyoming.

The FDA has only approved three drugs related to cannabis. One is the epilepsy drug Epidiolex, which is made from a highly purified form of CBD. The other two are lab-made medicines -- dronabinol (Marinol, Syndros) and nabilone (Cesamet) -- to treat nausea and vomiting from chemotherapy and low appetite in HIV patients.

The FDA has not approved any medical use of the cannabis plant (Cannabis sativa L.) itself or any of the many products made directly from the plant. Most medical marijuana involves using those products.

Despite many state laws allowing their use, these unapproved drugs also remain illegal under federal law. The U.S. Drug Enforcement considers cannabis a Schedule 1 drug, which means it has no medical use and a potential for abuse, just like heroin or LSD. Proposals to change that classification are under review. In the meantime, federal enforcement policies have varied from administration to administration.

If you use medical marijuana in a state where it's legal, being on the state's patient registry may give you some protection against arrest for possessing a certain amount of medical marijuana for personal use. The laws also often protect doctors who recommend marijuana use.

What about CBD products?

Some CBD products are legal, even under federal law. CBD is legal if it comes from hemp, which is any part of the cannabis plant with no more than 0.3% THC, the mind-altering substance in marijuana. That's why, in most states, you can buy foods, cosmetics, and other products containing CBD without a medical marijuana card. Keep in mind that while these products are legal, they aren't regulated or approved by the FDA.

The CDC says CBD can cause drowsiness, mood changes, diarrhea, and other side effects and is not safe during pregnancy.

Scientists who've reviewed medical marijuana data say we need much more information on benefits and risks for individuals, as well as on the broader public health impact of wide availability.

Research has been limited by federal laws. Scientists have to get special licenses from the federal government to study cannabis. Getting a license requires background checks, inspections, and special security and record-keeping procedures. Researchers also have to use cannabis from growers approved by the federal government, which might mean using products different from those used by most patients.

You can get medical marijuana in many states if you have certain conditions and a recommendation from a doctor. But, except for a few cannabis-related drugs approved by the FDA, these products have not undergone the rigorous study required of prescription drugs. They also remain illegal under federal law, though state laws give some protection to approved users.

Which strains of marijuana are used for medicine?

Marijuana dispensaries around the country offer products from many different strains of cannabis plants. While users often report different effects on their symptoms from different strains, research in this area is thin.

What might disqualify you from getting a medical marijuana card?

The rules vary by state, so you should check with your state's medical marijuana program. In general, you need to have a qualifying medical condition and a recommendation from a health care provider who participates in the state's program.