Despite how many Americans use cannabis—now about one in five—there has historically been a gap in our scientific understanding of its health effects. But as policies against the drug loosen, we’re starting to learn more than ever about cannabis.

You might think we know a lot about cannabis. Humans have been cultivating the plant for 12,000 years and using it as a psychoactive substance for at least 2,500. Conventional wisdom holds that cannabis offers a safe, natural alternative to pharmaceuticals for treating pain, nausea, muscle spasms, appetite, and post-traumatic stress disorder.

(Scientists discover the earliest direct evidence for humans using cannabis as a drug.)

And indeed there is evidence that cannabis can help with pain from cancer, and the U.S. Food and Drug Administration has approved two artificial cannabinoid medicines to help cancer patients with nausea and vomiting.

But now, we’re learning that the drug isn’t quite as harmless as some people believed. Thanks in part to loosening restrictions on accessing cannabis for medical research, scientists are revealing the downsides of using cannabis might be greater than we thought, posing long-term threats to the lungs, heart, gastrointestinal tract, and mental health.