One night in 2005, at a party at my house, two things happened: I had a single toke from a joint, and a friend introduced me to her new boyfriend. For most people this confluence of events would be no problem, but my body was not having it. As my friend's boyfriend dribbled on about his adventures in Peru, his fluffy hair began to morph and swirl. The more it swirled, the more I wanted to vomit. Then his voice started piping down through a tiny hole in the roof, then… nothing. It was lights out. That was the first time it happened, but soon enough it became apparent that this was my fate. I could not inhale marijuana—not even a little bit, not even sans alcohol—without blacking out. But why? Is my constitution so delicate, just a whiff of weed requires its total shut-down?
One of the only studies conducted on this phenomenon was published in 1992. Researchers from Duke University gave ten healthy men a strong joint to smoke while standing up, and reported that six participants felt "moderate" to "severe" dizziness. Those who experienced severe dizziness also showed marked decreases in blood pressure, which went as low as 60 mmHg.
The standing-up part is key because it indicates weed could bring on something called orthostatic hypertension, low pressure caused by the movement or position of the body.
"Marijuana can cause quite profound lowering of blood pressure, and cause users to faint as not enough blood gets to the brain," confirms Dr. Andrew Mongomery, a general practitioner. "A lesser lowering of blood pressure may lead to a sense of dizziness without actually passing out, [although] the biological mechanisms underlying this are highly complex and incompletely understood.
"Marijuana can also lead to anxiety," he adds, "with a secondary effect of dizziness—or act on the brain directly to create a sense of rotational dizziness."
Blood vessels dilate, causing the brain to be deprived of oxygen and thus blacking out.
Dr. Harry McConnell, a Professor of neuropsychiatry at Griffith University's Menzies Health Institute, says the reason behind a weed-related fainting spell depends on the situation and the individual. Aside from a possible drop in blood pressure, "it could also be seizures that cause blackouts, or the other chemicals mixed with it. After all, Marijuana is not pure, so it might have recreational chemicals mixed with it. People may not be aware of those ingredients.
"Marijuana might [also] cause vasodilation," Dr. McConnell continues, "where blood vessels dilate, causing the brain to be deprived of oxygen and thus blacking out. Sometimes episodes of vertigo can also cause blackouts."
Then there is the possible effect of other recreational substances, possible other drug interactions, personal medications, or medical conditions. A study from 2002 noted that while weed's "cardiovascular effects" are not associated with serious health problems for most young, healthy users, people with cardiovascular disease could be putting their health at risk. This is "because of the consequences of the resulting increased cardiac work, increased catecholamine levels, carboxyhemoglobin, and postural hypotension."
Whatever the reason behind your body's hatred of the herb, accept that the weed life may not be for you, and get yourself seen to. "[Fainting from marijuana use] is not certainly uncommon," confirms McConnell. "It's recognized. But it's always important to go to a doctor and get evaluated."