Health insurance companies are not covering medical marijuana, which can cost up to $1,000 monthly, according to the Los Angeles Times.

Susan Pisano, a spokeswoman for America’s Health Insurance Plans, an industry trade group, says insurers do not have enough evidence that marijuana is safer and more effective than other treatments.

While medical marijuana is legal in 21 states, the drug is still outlawed by the federal government. It is also not approved by the Food and Drug Administration (FDA). Most insurance companies will not cover treatments not approved by the FDA, the article notes. Drug approvals require large clinical studies to measure their safety, effectiveness and side effects. These studies can take years and cost millions of dollars.

Studying marijuana poses additional challenges. The drug is considered a Schedule I drug under the federal Controlled Substance Act, meaning it is considered to have a high potential for abuse, and no accepted medical use. Under the law, extra precautions are needed to study marijuana. Any study would have to be approved by the FDA and might be reviewed by the Public Health Service. The Drug Enforcement Administration would have to issue a permit, after ensuring the researchers have a secure place to store the marijuana. The National Institute on Drug Abuse (NIDA) would also review the study plan.

Any study would have to use marijuana supplied by NIDA, which limits the strains available for study. Finding a location to conduct the study could also pose problems, the article notes.

If the FDA does approve medical marijuana, coverage of the drug may not be widespread. Large companies that pay medical bills for their employees and their families choose what their insurance plan covers. They may decide they don’t want to foot the bill for medical marijuana coverage.