A growing number of people in the United States are enrolling in medical marijuana programs, with the total surpassing 2.97 million through 2020, more than quadrupling the number of people enrolled in 2016, according to a study published in Annals of Internal Medicine.

These programs enable participants to buy marijuana for medical use. Although marijuana is still illegal under the federal Controlled Substances Act, the National Conference of State Legislatures reports that 37 states, D.C. and four U.S. territories had legalized marijuana for medical use as of February.

State laws vary on which health conditions qualify someone for participation, but the study’s researchers found that the most common condition cited by current participants is chronic pain (noted by about 61 percent of enrollees), followed by post-traumatic stress disorder (11 percent).

Other conditions that may qualify people for participation, depending on their state’s law, include multiple sclerosis, Alzheimer’s, AIDS, glaucoma, Crohn’s disease and chemotherapy-induced nausea and vomiting.

To enroll in a medical marijuana program, people must get a doctor’s referral and sign up on their state’s registry. Then, for a fee, they are given a card that allows them to buy medical marijuana from an approved dispensary. It may be available in a variety of forms: pill, powder, liquid, oil and dried leaves. [Read more at The Washington Post]