Now that cannabis has become widely available across much of the United States, the scientific community is finally getting a clearer picture of what heavy long-term use might actually be doing to our brains. And according to a massive new review published in the Journal of Psychiatric Research, one of the clearest patterns emerging is a strong connection between cannabis use disorder and major depression.

The study analyzed 55 previous studies involving more than 3.2 million people and found that nearly 32 percent of people with cannabis use disorder, or CUD, also experienced major depressive disorder, more commonly known as MDD. On the flip side, just over 10 percent of people with depression also met the criteria for problematic cannabis use.

That might not sound shocking at first glance, but those numbers are dramatically higher than what researchers see in the general population.

Complicating matters is that scientists still don’t fully understand why this is happening. Is it a chicken and the egg situation? Are people with depression self-medicating with cannabis? Or is heavy cannabis use increasing the risk of depression? A lot more research is needed to figure that out. And then there is the third option: are both conditions being driven by the same underlying factors, like genetics, trauma, stress, or a wide variety of other mental health issues?

The go-to temporary answer is that it’s probably a combination of all three.

No One’s Arguing That Cannabis Use Absolutely Causes Depression

The review also found that the numbers are even more intense among people being treated for depression in clinical environments. But 28 percent of said patients also showed signs of cannabis use disorder. Researchers think that’s an important detail since the symptoms can blur together in ways that make a clear-cut diagnosis near impossible.

On top of that, cannabis withdrawal can produce its own host of depressive symptoms, like anxiety, sleep problems, irritability, and steep emotional lows that can masquerade as depression itself.

The researchers aren’t arguing that using cannabis automatically causes depression. It’s just forming a strong association and not direct causation.

It’s ultimately a good thing that cannabis has been legalized, as it expands personal freedoms, tax revenues, and is slowly addressing a major gap in the criminal justice system. However, its rapid pervasiveness has led to a series of observations from mental health professionals that are going to help us in the long run as we navigate all this new science to help map out the psychological trade-offs of readily available cannabis.