Cardiovascular Effects on the heart and blood vessels are marked by a 20-50% increase in the heart rate and a mild decrease in blood pressure, though the documented effects of cannabis on fluid pressure are inconsistent. Pressure within the blood vessels appears to decrease mildly with continued use (mild hypotension), but some research has shown the opposite effect (mild hypertension). The heart rate (beats per minute) increases slightly for up to 30 minutes following smoked inhalation and longer for oral ingestion. These effects subside over the following 2 to 4 hours. In chronic long-term use, a slightly depressed heart rate and slight lowering of blood pressure (hypotension) has been observed.
These distinct cardiovascular effects of cannabinoids have been the focus of many debates among scientists. Peter Nelson, in the employ of the Criminal Justice System of Australia, noted the subjective nature of certain researchers’ scientific evaluations:
Jones claims that THC has “far more effect (on the cardiovascular system) than nicotine,” but fails to tell us how. In fact, the findings of Benowitz and Jones he presents on long term oral administration of THC (above) shows an effect which could be construed as potentially useful in combating the cardiovascular effects of long term stress. As is often the case in THC research, interpretation is in the eye of the beholder.
Nelson also informed the Australian government that, “His [Jones’] statements comparing nicotine with THC are particularly ill founded.”.
In a 1979 paper titled, Effect of marijuana on cardiorespiratory responses to submaximal exercise, it was reported that:
Smoking [marijuana] had no effect on blood pressure, pulmonary ventilation and oxygen uptake, but did include a marked increase in heart rate which persisted throughout exercise and recovery periods, averaging 34% higher than control values at rest, 18% higher during exercise, and up to 50% higher during recovery.
In 1997, the United Nations World Health Organization report found no cause for alarm:
The conclusion reached by the Institute of Medicine in 1982 still stands: the smoking of marijuana causes changes to the heart and circulation tha are characteristic of stress… [but] there is no evidence… that it exerts a permanently deleterious effect on the normal cardiovascular system.
There is no reason to suspect that a slight increase of heartbeats per minute caused by cannabis use is any more injurious to the heart or other organs than normal physical exercise. A small increase in the heart rate may be considered beneficial to the entire body in allowing greater blood flow. In contrast, long-term chronic use is usually associated with a slight decrease in the heart rate. A review of research by the Australian National Drug Strategy revealed that, “Tolerance to the cardiovascular effects develops within seven to ten days in persons receiving high daily doses [of THC].”
Both field studies and electrocardiographic research have failed to detect cardiac pathology caused by cannabis use. However, as a precaution, patients suffering from acute cardiac disease, such as angina pectoris, are usually advised by their physician to abstain from using marijuana.
One other notable vascular effect of cannabis is typically displayed in the dilation of blood vessels, causing a reddening of the eyes. The cannabis-induced “red-eye” symptom represents a general increase in cerebral blood flow. The mild effect is temporary and is not considered injurious.