Is weed actually bad for you?

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The legalization and regulation of marijuana use has long been a subject of national debate. Since the legalization in Washington and Colorado, many are closely watching whether other states will follow suit. Supporters of legalization have argued that it could provide tax revenue and reduce drug smuggling. As a medical researcher, I am interested in how science might help inform this debate. I wanted to look at whether or not marijuana actually has detrimental health effects. It is a drug after all, right? Yet marijuana has also been used medicinally to relieve pain, so is it really bad for us? I set out to learn more about our current knowledge of the drug’s effects on the human mind and body.

Marijuana and Mental Health

Is there evidence that smoking marijuana impairs mental health? An article in the British Medical Journal looked at this question in a group of 759 people in New Zealand (Arseneault et al., 2002). These individuals were all born in 1972 or 1973 and provided health data throughout their lives. The study measured drug use at ages 15 and 18 and then followed up on mental prognosis as defined by symptoms and diagnoses of depression and schizophrenia at age 26. They also controlled for psychotic symptoms at age 11 in case drug use was an effect of mental illness rather than a cause. This study did find a positive correlation for adolescent marijuana use and adult schizophrenia but not depression. The youngest cohort was most vulnerable. However, it should be noted that the prevalence of schizophrenia was still only 10% in the 15-year-old drug user group compared to 3% in the control group. The researchers advocated for further study in larger groups. This study suggests that marijuana may have detrimental effects on the brain, but I wanted to understand more about the mechanism of the drug at a neurological level.

Cannabinoids and the Brain

Valesco et al. 2012

The psychoactive ingredient in marijuana is delta-9-tetrahydrocannabinol or THC, which belongs to class of compounds called cannabinoids (Wilson and Nicoll, 2002). THC binds to a receptor in the brain known as CB1. This receptor is present in many locations in the central nervous system including the cortex, hippocampus, and basal ganglia where it is involved in modulating pain, appetite, and memory. Our brains have this receptor because there are compounds known as endocannabinoids with similar structures to THC that are found naturally inside the body and that bind to the same receptor.

Signals propagate in the brain when chemicals known as neurotransmitters are passed from one neuron (presynaptic) to another (postsynaptic) across a divide called the synaptic cleft. Cannabinoids are called retrograde messengers because they travel against the direction of the signal before binding to CB1 on the first neuron. Binding functions to decrease calcium levels in the cell, thereby calming down the signal transmission.

If marijuana can calm down pain signaling, this explains why it has been successful medicinally as a pain killer. A recent study of patients with fibromyalgia, a type of chronic pain, reported relief from pain, joint stiffness, and insomnia after using marijuana for two hours (Fiz et al., 2011). 

Studies in rodents have found that THC can lead to neurotoxicity, but dosage and duration were important. Toxicity did not develop until after the equivalent of 7 to 10 years of usage in humans (Scallet, 1991).

Marijuana and the Immune System

It seemed logical to me that marijuana would have a role in the brain, but I was surprised to find that it also functions to regulate the immune system. This happens because there is a second receptor that responds to cannabinoids called CB2 that is found on the cells of the immune system (Rieder et. al., 2010). Multiple studies have shown that cannabinoids are immunosuppressive. This can be very good for people whose immune systems are too active, and can be used to treat inflammation in multiple sclerosis, diabetes, and rheumatoid arthritis. However, in high doses these compounds can increase susceptibility to infection.

Conclusions and Personal Opinions

So is marijuana actually detrimental to human health based on science? I would say that for people who start smoking during adolescence and continue using the drug heavily for a number of years the answer is yes. However, small doses can be beneficial, particularly for people needing treatment for chronic illnesses of the nervous or immune systems. There is also talk of using cannabinoids as anti-cancer therapy, although I did not go into that here.

References

1. Arseneault L., Cannon M., Poulton R., Murray R., Caspi A., Moffitt T. E. (2002). Cannabis use in adolescence and risk for adult psychosis: longitudinal prospective study. BMJ. 325, 1212–1213.

2. Wilson, R. and R. Nicoll. (2002). Endocannabinoid Signaling in the Brain. Science.296: 678.

3. Fiz J, Durán M, Capellà D, Carbonell J, Farré M. Cannabis use in patients with fibromyalgia: effect on symptoms relief and health-related quality of life. PLoS One. 2011;6(4):e18440

4. Scallet, Andrew. (1991). Neurotoxicology of cannabis and THC: A review of chronic exposure studies in animals. Pharmacology Biochemistry and Behavior. 40(3): 671–676.

5. Rieder, S. A.  Chauhan, A., Singh, U., Nagarkatti, M.,

Nagarkatti. P. (2010). Cannabinoid-induced apoptosis in immune cells as a pathway to

immunosuppression. Immunobiology. 215(8): 598–605.

6. Velasco, G., Sánchez, C., Guzmán, M. (2012). Towards the use of cannabinoids as anti-tumor agents. Nature reviews cancer. 12: 436.

7. Marijuana frond picture. Mass Firearms Attorney. Marijuana and Firearms Licensing. 7 May 2013. www.massguns.com.