A review published in the Journal of the American College of Cardiology this month suggests that an excess of 2 million patients with cardiovascular disease also use marijuana.

The research, penned by Brigham and Women's Hospital cardiologist Muthiah Vanduganathan, MD, MPH, and colleagues, comes on the heels of cannabis legalization in Illinois and at a time when recreational marijuana use is climbing fast across the U.S. Despite that, the authors said, there’s still a lot we don’t know about how the drug affects human health.

What we do know about marijuana’s effect on CV wellness—like the fact that inhaling cannabis can raise a person’s heart rate and lower their blood pressure—is largely limited to observation. Marijuana is classified as a schedule I drug by the U.S. Drug Enforcement Agency, so its use in research settings is highly restricted and subject to approval from local, state and federal agencies. Right now, randomized clinical trials dealing with cannabis just aren’t feasible.