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Marijuana has been shown to help ease pain and a few other health problems. Yet two-thirds of U.S. states have decided pot should be legal to treat many other conditions with little scientific backing.

At least 1.4 million Americans are using marijuana for their health, according to an Associated Press analysis of states that track medical marijuana patients. The analysis is based on data from 26 states and the District of Columbia.

The number of medical marijuana cardholders more than tripled in the last five years as more states jumped on the bandwagon. States that expanded the use of medical pot for common ailments such as severe pain, post-traumatic stress disorder and anxiety saw a boost in enrollment, the AP found.

The U.S. government, meanwhile, considers marijuana as both an illegal drug and a therapeutic herb worth more study.

Here’s a look at the health claims and research on medical marijuana to date:

The proof

Besides chronic pain, there’s strong evidence marijuana, or its ingredients, can ease nausea and vomiting from chemotherapy and help with symptoms of multiple sclerosis.

Several European countries have approved Sativex, a mouth spray containing THC and CBD, for multiple sclerosis symptoms. British drugmaker GW Pharmaceuticals is seeking U.S. approval for Sativex. (Note: THC is the natural ingredient that causes marijuana’s mind-altering effect. CBD, which is also found in regular hemp, doesn’t get people high.)

Last year, the U.S. Food and Drug Administration approved Epidiolex, made from CBD, to treat two rare seizure disorders. As a result, other companies are pursuing FDA backing for products based on marijuana ingredients.

Prescription drugs already on the market use synthetic THC to treat weight loss, nausea and vomiting in patients with AIDS or cancer. And researchers continue to study whether marijuana helps with PTSD, back pain and other problems.

An opioid alternative?

New York, New Jersey, Pennsylvania and New Mexico allow medical marijuana for opioid addiction despite little evidence that it works.

But marijuana may be helpful in reducing the use of opioid painkillers. The National Center for Complementary and Integrative Health, better known for its research on herbs and yoga, has set aside $3 million for studies to determine which of marijuana’s 400-plus chemicals help with pain.

THC was excluded, however. Its mood-altering effects and potential for addiction and abuse make it less useful to relieve pain, said Dr. David Shurtleff, the agency’s deputy director. And THC has been studied more than the lesser-known compounds.

Cure for cancer?

Despite online claims, there’s only weak evidence that marijuana’s ingredients might one day be used to fight cancer. Most studies have been on animals or in the lab, and results have been mixed.

In one study, nine patients with an aggressive form of brain cancer had THC injected into their tumors; any effect on their survival was unclear.

Another study found worrying evidence that marijuana might interfere with some cancer drugs, making them less effective.

Research is stymied by laws

The U.S. government grows marijuana for research at a farm in Mississippi and generally bans grant-funded studies of real-world products.

With increased demand by researchers for pot to be used in testing, the Drug Enforcement Administration created an application process for growers but has not acted on more than two dozen applications.

Such challenges are common for scientists studying an outlaw medicine, said Dr. Igor Grant, who directs the oldest marijuana research center in the U.S. at the University of California, San Diego.

There, scientists are studying marijuana chemicals for autism and brain disorders.

Established by state law in 2000, UCSD’s Center for Medicinal Cannabis Research once relied solely on California for funding. The center now has support from private foundations, a sign of growing public acceptance of the research.

Studying cancer pain

Minnesota medical marijuana patients must regularly fill out surveys about their symptoms and side effects. That allows researchers to study how people with cancer react to marijuana.

In one study, a third of cancer patients made only one purchase and didn’t come back during a four-month period. They may have died, or decided marijuana was too expensive or didn’t work.

Of the rest, most reported improvements in vomiting, pain, disturbed sleep, anxiety and depression with few side effects.

Marijuana can ease many symptoms “all at one time,” but more study is needed, said study co-author Dr. Dylan Zylla of the healthcare system HealthPartners. He has no financial ties to cannabis companies.

Zylla is studying whether cancer patients can decrease their prescription opioid use while using marijuana.

Marijuana “does seem to help patients,” he said, “but so much is unknown about the risks, side effects and drug interactions.”

—AP