Cardiovascular effects of cannabis

The most consistent effect of cannabis in humans and animals is an increase in heart rate (Perez-Reyes, 1990). This effect parallels the experienced ‘high’ and is related to the dose of THC (Perez-Reyes, 1990; Chesher and Hall, 1999). The hearts of healthy young adults will only be mildly stressed by the acute cardiovascular effects of cannabis (Tennant, 1983).

An increased heart rate is most pronounced in occasional cannabis users because the increased heart rate decreases with regular cannabis use as users become tolerant to the effects of THC (Chesher and Hall, 1999; Sidney, 2002). Tolerance to these cardiovascular effects develops within 24 hours in laboratory studies and, in some cases, even large amounts of cannabis had little effect on heart rate (Benowitz and Jones, 1975). Tolerance to these effects has also been observed in chronic heavy cannabis users in Costa Rica (Carter, 1980), Greece (Stefanis et al., 1977), and Jamaica (Rubin and Comitas, 1975).

There are a number of concerns about the effects of cannabis use on patients with ischaemic heart disease, hypertension, and cerebrovascular disease (Jones, 2002; Sidney, 2002). These include the possibilities that cannabis use may cause cardiac arrhythmias, chest pain, and myocardial infarction (or heart attack) that may not be noticed because the analgesic effects of THC may mask chest pain and delay treatment-seeking. Cannabis smoking also increases the level of carboxyhaemoglobin in the blood, decreasing the amount of oxygen reaching the heart, increasing its work and, perhaps, increasing atheroma formation. Patients with cerebrovascular disease could have strokes induced by changes in blood pressure and patients with hypertension could experience exacerbations of their disease for the same reason (Chesher and Hall, 1999).

A number of laboratory studies have found adverse effects of smoking cannabis cigarettes on patients with occlusive heart disease. Aronow and Cassidy (1974) compared the effect of smoking a cannabis and a placebo cigarette on heart rate and the time required to induce chest pain in an exercise tolerance test. Heart rate increased by 43%, and the time taken to produce chest pain halved after smoking a cannabis cigarette. Aronow and Cassidy (1975) compared the effects of smoking a single cannabis cigarette and a high nicotine cigarette in 10 men with occlusive heart disease, all of whom were cigarette smokers. Smoking cannabis produced a 42% increase in heart rate, compared with a 21% increase after smoking the tobacco cigarette.