Three senators, two Democrats and a Republican, introduced a bill on Tuesday that would allow patients to use marijuana for medical purposes in states where it is legal, without fear of federal prosecution for violating narcotics laws.

The bill makes a number of important changes to federal marijuana policies — and it deserves to be passed by Congress and enacted into law. Though this legislation would not repeal the broad and destructive federal ban on marijuana, it is a big step in the right direction.

The most important change would reclassify marijuana from a Schedule I drug under the Controlled Substances Act, which is intended for drugs, like heroin, that have no accepted medical use in the United States, and place it instead in Schedule II, the classification for drugs that have a legitimate medical use but also have a “high potential for abuse.”

The Schedule I classification made no sense because there is a medical consensus that patients with AIDS, cancer, epilepsy and serious degenerative conditions can benefit from marijuana. And millions of patients have used marijuana to relieve pain, nausea, appetite loss, insomnia and seizures associated with various illnesses.