|MadSci Network: NeuroScience|
The effects of marijuana are very controversial because research has shown inconsistent or contradictory findings. In addition, results are not always consistent across subjects and it is known that expectancy and environmental cues often influence the effects of marijuana. Lastly, its effects can vary widely depending on the concentration of the drug. With this in mind, this is what we know. The primary active ingredient in marijuana is delta-9-tetrahydrocannabinol or THC. THC is extremely lipid- soluble so it is distributed to virtually all areas of the body. Recently, a receptor of THC was isolated and these seem to be concentrated primarily in the cortex, hippocampus, cerebellum and basal ganglia (this would suggest some effects on learning and motor movements). In addition, at least one endogenous cannabinoid has also been identified.
Marijuana causes blood-shot eyes which results from a dilation of the small blood vessels in the whites of the eyes. However, it does not cause dilation of pupils. A therapeutic effect of marijuana is that it decreases intraocular pressure (often by as much as 30%), which is why it may be useful as a treatment for glaucoma. A common effect is dry mouth, with a strong urge to drink liquids. Another is something that is commonly called the “munchies”. The munchies begin several hours after smoking marijuana, but this effect declines after a few weeks in continuous users, and in these cases, appetite is actually depressed. Smoking marijuana typically leads to an increase in heart rate and fluctuations in blood pressure and THC can act as both an antiemetic and anti-convulsant.
THC has a sedating, relaxing and anti-anxiety effect. This general feeling of well being is often called “being high”. At high concentrations, disinhibition may be seen and there are apparently some deficits in short-term memory. For example, it may be difficult to memorize a new list of words. Also, at high concentrations of THC, some people show something called temporal disintegration. In other words, they have problems holding information in the brain and then using it. A common example, is that a person will begin a sentence and then forget what he was going to say. At high doses, THC can act as an hallucinogen (although this is not commonly seen). In addition, marijuana seems to decrease attention and increase distractibility.
Smoking marijuana can induce psychosis in certain populations (such as persons with schizophrenia), but other drugs (including alcohol) can do so as well. It does not seem to cause psychosis in the normal population. At high doses, it may cause anxiety, paranoia or panic attacks. There have been isolated reports of “cannabis dementia” after prolonged and heavy use, but such results are inconsistent. There is some good evidence that marijuana impairs driving, but this seems more related to a decrease in attention rather than a decrease in reaction time. At low doses, marijuana may cause drowsiness, but at higher doses it seems to interfere with sleep. Specifically, it tends to decrease total REM time and also decrease eye movement during REM sleep.
Although tolerance has been shown in laboratory animals, this effect is less clear in humans. In fact, there are many reports of a reverse tolerance. In other words, chronic users may be more sensitive to the effects of THC rather than less sensitive. Research results are not conclusive, but generally, tolerance does not seem to occur. Similar inconsistent results are seen concerning withdrawal. There is some fairly good evidence suggesting that withdrawal does occur with high doses of THC, but not with low ones.
There is no evidence that marijuana increases violence or aggression. There is no clear evidence that marijuana produces the so-called amotivational syndrome (e.g., greater introversion and reduced motivation), but some studies report a negative correlation between grade point average and marijuana use. There is a positive correlation between marijuana and other drug use, but no strong conclusive evidence for the “stepping stone” effect. In other words, that smoking marijuana will lead to the use of other drugs. Marijuana seems to lower testosterone in both human and nonhuman males, but it is not clear whether this actually reduces sex drive or fertility. There is evidence to suggest that marijuana interferes with our body’s immune response, which may increase certain infections and illnesses, but the connection between smoking marijuana and cancer is still open to question.
I hope this helps. For more information about marijuana and its effects see W.A. McKim, Drug and Behavior (3rd edition, 1997, Prentice Hall); or J. Diaz, How Drugs Influence Behavior (1997, Prentice Hall).
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