THC disrupts the nerve cells in the part of the brain where memories are formed. This makes it hard for the user to recall recent events (such as what happened a few minutes ago), and so it is hard to learn while high. A working short-term memory is required for learning and performing tasks that call for more than one or two steps. Some studies show that when people have smoked large amounts of marijuana for many years, the drug takes its toll on mental functions. Among a group of long-time heavy marijuana users in Costa Rica, researchers found that the people had great trouble when asked to recall a short list of words (a standard test of memory). People in that study group also found it very hard to focus their attention on the tests given to them. It may be that marijuana kills some brain cells. In laboratory research, some scientists found that high doses of THC given to young rats caused a loss of brain cells such as that seen with aging.
Doctors advise pregnant women not to use any drugs because they might harm the growing fetus. Some scientific studies have found that babies born to marijuana users were shorter, weighed less, and had smaller head sizes than those born to mothers who did not use the drug. Smaller babies are more likely to develop health problems. Other scientists have found effects of marijuana that resemble the features of fetal alcohol syndrome. There are also research findings that show nervous system problems in children of mothers who smoked marijuana. Researchers are not certain whether a newborn baby's health problems, if they are caused by marijuana, will continue as the child grows. SIDE EFFECTS:
Most recent research on the health hazards of marijuana concerns its long-term effects on the body. Studies have examined the brain, the immune system, the reproductive system, and the lungs. Suggestions of long-term damage come almost exclusively from animal experiments and other laboratory work. Observations of marijuana users and the Caribbean, Greek, and other studies reveal little disease or organic pathology associated with the drug. For example, there are several reports of damaged brain cells and changes in brain-wave readings in monkeys smoking marijuana, but neurological and neuropsychological tests in Greece, Jamaica, and Costa Rica found no evidence of functional brain damage. Damage to white blood cells has also been observed in the laboratory, but again, its practical importance is unclear. Whatever temporary changes marijuana may produce in the immune system, they have not been found to increase the danger of infectious disease or cancer. If there were significant damage, we might expect to find a higher rate of these diseases among young people beginning in the 1960s, when marijuana first became popular. There is no evidence of that. The effects of marijuana on the reproductive system are a more complicated issue. In men, a single dose of THC lowers sperm count and the level of testosterone and other hormones. Tolerance to this effect apparently develops; in the Costa Rican study, marijuana smokers and controls had the same testosterone levels. Although the smokers in that study began using marijuana at an average age of 15, it had not affected their masculine development. There is no evidence that the changes in sperm count and testosterone produced by marijuana affect sexual performance or fertility. In animal experiments THC has also been reported to lower levels of female hormones and disturb the menstrual cycle. When monkeys, rats, and mice are exposed during pregnancy to amounts of THC equivalent to a heavy human smoker's dose, stillbirths and decreased birth weight are sometimes reported in their offspring. There are also reports of low birth weight, prematurity, and even a condition resembling the fetal alcohol syndrome in some children of women who smoke marijuana heavily during pregnancy. The...