"Not a bad set of questions for a pothead," says Dr. Jim Pfaus who teaches the neurobiology of sexual behaviour at Concordia University, "but I am sure he would like two heads working better than one, so first things first. The active ingredient in marijuana is tetrahydrocannabinol (THC). This binds to cannabinoid Type 1 receptors in the brain to induce a sedative-hypnotic effect. Such effects occur endogenously after orgasm and indeed the sedation after orgasm helps produce an anxiolytic effect along with outflow that prevents getting an erection too quickly in men."

Basically what Pfaus is saying, Lazy, is that erectile dysfunction is totally common in men who are chronic pot smokers, "especially if they are steady-state users, meaning they always keep a basal amount of THC in the system. The idea of having a partial erection is entirely consistent."

As for boner-enhancing pharmaceuticals, also known as PDE5 inhibitors, Pfaus says the three on the market — sildenafil (Viagra), tadalafil (Cialis) and vardenafil (Levitra) — "are all roughly the same in terms of potency. Vardenafil and tadalafil last longer than sildenafil, but that's about it." All of these pills have the same effect: they relax the smooth muscles lining blood vessels that take blood into the corpora cavernosa of the penis, where blood is held for erection.

"Theoretically, any PDE5 inhibitor should be good at stimulating erection in men with a chemical imbalance that favours the inhibitory side of things," says Pfaus. "There are a few papers out there showing that pot-smoking men are able to obtain and sustain more rigid erections with PDE5 inhibitors and this interaction may be part of the motivation for their recreational use."

Pfaus cites only one paper that shows an adverse interaction of cannabis and sildenafil, where the combination possibly induced a heart attack in a young man.

"High doses of THC can inhibit the cytochrome P450 3A4 isoenzyme, which is critical in the generation of calcium currents in the heart muscle, keeping the heart beating rhythmically," he says. "Because sildenafil relaxes smooth muscle around blood vessels, it can potentiate the effect of THC in the inhibition of those calcium currents, thereby reducing the ability of the heart to beat rhythmically and, in the case of that one young man, perhaps inducing the heart attack. I say perhaps because it was a clinical case report and there was much discussed in it about other potential predisposing factors. If there were a real connection between cannabis and heart attack induced by Viagra, there would be way more papers in the literature on it. The fact that there aren't suggests that predisposing factors, such as a family history of heart disease, is more likely the causal factor."

In the end, Pfaus says it's a pretty safe combination but that if you have a family history of heart disease you may want to be a little careful. "Or alternatively, he could just kick the habit and watch his erections return with a vengeance."

Girls, gurls, grrrls