A recently published study in Journal of the American Heart Association by Jouanjus et al. (2014) concludes cannabis is a possible risk factor for cardiovascular-related events:
Cannabis use: signal of increasing risk of serious cardiovascular disorders
However, it should be noted that this study from France is based on only 35 cases of cannabis-related cardiovascular events out of a total of 1979 cannabis-related reports. Here, using the total cannabis-related reports as the denominator, the authors report an increase in the proportion of cardiovascular-related events from 2006-2010 (1.1% to 3.6%). Yet, considering the number of regular cannabis users reported by the authors (1.2 million), 35 cases does not seem to be strong evidence of a cardiovascular risk.
Why is there concern over cannabis smoking and adverse cardiovascular outcomes? Smoking marijuana causes an increase in heart rate and dilates (widens) blood vessels (Ashton, 2001). The widening of blood vessels in the conjunctiva of the eye is what produces the characteristic reddening seen in intoxicated smokers. The dilation of blood vessels may also cause a drop in blood pressure (hypotension, the opposite of high blood pressure, i.e., hypertension), and this may cause dizziness or fainting (Jones, 2002). By contrast, blood vessels in the fingers and toes may constrict, causing tingles or cold sensation in the extremities. After repeated marijuana use tolerance to these effects occur, which can also be lost rapidly after marijuana cessation (typically after 48 hours). Nevertheless, THC is fat-soluble (meaning it can be stored in fat cells), and THC can continue to be released into the bloodstream in detectable levels for up to a month (Ashton, 2001).
Does smoking marijuana cause cardiovascular-related harms? One important distinction for this question is the main cardiovascular-related harm studied. The events of main interest are those that disrupt or block blood flow to the heart (i.e., a heart attack, also called a myocardial infarction) or brain (i.e., a stroke), and abnormal changes to heart rhythm (i.e., palpitations, also called arrhythmias). Another important distinction is whether these cardiovascular-related events can be caused by the immediate use of marijuana, and/or the result of an accumulation of cardiovascular insults over time due to chronic marijuana consumption.
Several reviews have looked at the subject (Jones, 2002; Sidney, 2002; Aryana and Williams, 2007; Debois and Cacoub, 2013). These reviews concluded that marijuana poses a risk to older individuals, especially those with cardiovascular disease or at risk for such cardiovascular-related events. However, much of the evidence comes from case reports or clinical samples of patients where a cardiovascular event has already occurred (Mittleman et al., 2001; Mukamal et al., 2008; Wolff et al., 2011). These types of studies may not be representative of the risk in the overall population.