MARIJUANA has been used in human societies for over 6,000 years. For much of the last 100 years it has been viewed as a drug of abuse, its use restricted and illegal in much of the world.

However, in many countries including ours it has remained popular for recreational, medical, and religious uses. More recently there has been a trend worldwide towards legalising the use of marijuana for personal consumption. Along with this change studies have found medical uses for marijuana and proven it’s relative safety for recreational consumption particularly when compared to other drugs of abuse. There have, however, been signals of potential harm for the heart and vascular system from the use of marijuana. It’s important to note that given the grey legal status of marijuana use at the federal level in the United States and in many European countries, it is difficult for researchers to conduct large safety studies as with legal drugs. However, large-scale observational studies involving thousands of patients have begun to appear and have been presented at cardiology meetings this year.

When we speak of the use of marijuana, we are not referring to one chemical agent but rather hundreds of chemicals that are present in the plant which have the potential to cause physiological effects. Currently cannabinoids are thought to be responsible for most of the effects on the brain and more than 100 of these compounds have been identified. The two most well studied are delta-9-tetrahydrocannabinol (THC) and cannabidiol (CBD). THC appears to be responsible for more of the psychoactive effects of marijuana while CBD is thought to help with relaxation and pain control. Marijuana can be cultivated to have higher or lower absolute and relative levels of THC and CBD which can influence the overall effect of the plant on the user.

Beneficial effects of marijuana

Multiple studies have documented that the use of marijuana can have beneficial effects. These include pain relief, particularly in neuropathic pain, cancer-related pain and fibromyalgia. Marijuana also improves symptoms of nausea and vomiting and can be useful in patients who are receiving chemotherapy. It has been used in the treatment of spasticity in multiple sclerosis and for some types of epilepsy. Our own University of the West Indies developed a medicine for glaucoma using extracts from the marijuana plant. There is some evidence that marijuana may be useful in the management of opioid withdrawal. The use of marijuana has been proposed in several other diseases for which the evidence of significant benefit is not clear cut, including anxiety/depression, Alzheimer’s disease, cancer, inflammatory bowel disease, Parkinson’s disease, and sleep disturbances.

Physiological effects of marijuana on the heart and vascular system

The chemicals in the marijuana plant have many effects on the heart and vascular system. It is important to realise that most of the data that we have in this regard comes from the study of THC and CBD. The effects of the other cannabinoids are not well understood. Our bodies produce several cannabinoids that have physiological effects and thus we have receptors through which these endogenous cannabinoids act. These receptors can be affected by THC and CBD. The interactions can be quite complex with THC acting directly on receptors and CBD playing more of a regulatory role and modifying the effect of THC or other cannabinoids that are produced in the body. Cannabinoid receptors are present in many tissues including the heart and vascular system, the gastrointestinal tract, the immune system, and the brain. The effects on the heart and vascular system can be quite varied depending on the route of administration. (smoking vs ingestion), the relative ratio of THC vs CBD that is absorbed and the potency or the amount of cannabinoids that are taken.

A variety of physiological and pathological effects have been described because of the binding of THC and CBD. THC has been noted to activate the fight or flight system, cause abnormal growth in heart muscle and vascular cells, increase inflammation in heart vessels and to increase toxic chemicals in the heart and blood vessels. Whereas CBD has been noted to suppress the activation of inflammatory cells and chemicals. These somewhat antagonistic effects have led some to hypothesise that THC increases the risk of heart and vascular disease while CBD attenuates it.

Acute clinical effects of marijuana use on the heart

Aside from the psychological effects, marijuana has acute effects on the heart and vascular system which can be complicated given the different effects of THC and CBD. When the effect of THC predominates (as in most patients who are using marijuana for its euphoric effects), there is an increase in heart rate and blood pressure. The amount of oxygen required by heart muscle increases and there is evidence for an increase in abnormalities in the vessel walls with greater risk of clot formation. In patients using marijuana with higher CBD effects the heart rate and blood pressure fall and blood vessels dilate. In some diabetic models the CBD effect results in reduced vessel inflammation.

General concerns for the heart and blood vessels

The absence of large gold standard studies, the multiple ways in which marijuana is used and the fact that there is a great variety of strains with differing levels of THC and CBD along with limited data on other cannabinoids present in the plant make definitive general statements challenging. One trend that has been clear is the rise in the amount of THC which is present in marijuana plants. This has been well documented in the studies in the United States. Data from a study in Colorado found that in the 1970s the average THC concentration in the marijuana plant was less than three per cent. Currently levels of 14 per cent can be found in some varieties. Given the cardiac and vascular effects of THC this potentially may be a clinical issue. Another concern is the effects of combustion on the chemicals in the marijuana plant in those who smoke marijuana. It has been documented that the amount of carboxyhemoglobin (a protein that interferes with the distribution of oxygen throughout the body) increases by five-fold after smoking. Tar, which is a chemical produced by combustion of plant materials and which has been well studied in cigarettes, is also present in smoking marijuana rising by three-fold from baseline. Tar has been associated with lung damage, vascular damage and cancer risk in cigarette smokers and may well be an issue in those who smoke marijuana.

Aside from these general concerns, studies have been done over many years raising the question of heart and vascular damage in those who smoke marijuana. These initial reports were generally small and short term. However, larger observational studies are now being conducted and reported. We will review these clinical studies in our article next week.

Dr Ernest Madu, MD, FACC and Dr Paul Edwards, MD, FACC are consultant cardiologists for Heart Institute of the Caribbean (HIC) and HIC Heart Hospital. HIC is the regional centre of excellence for cardiovascular care in the English-speaking Caribbean and has pioneered a transformation in the way cardiovascular care is delivered in the region. HIC Heart Hospital is registered by the Ministry of Health and Wellness and is the only heart hospital in Jamaica. Send correspondence to info@caribbeanheart.com or call 876-906-2107.