Research into the effects of recreational drugs on cognition is hampered by several factors, most especially the fact that use of a drug is usually confounded with the taking of other recreational drugs, including nicotine and alcohol, and other lifestyle factors that may in themselves be associated with impaired memory. Moreover, while you can try your hardest to match your drug users with controls that are like the users in all aspects except that of substance abuse, without neurocognitive data on their brains and cognitive performance before they began taking drugs, you cannot be sure that any damage or impairment seen is a consequence, and not a precursor.
In the light of these problems, it is not surprising that the evidence for the effects of recreational drugs on cognition is inconsistent.
Those who use cannabis (as opposed to having used it in the past) may be more likely to have short-term memory problems.
Long-term use of marijuana has been found to affect verbal fluency, verbal memory, attention, and psychomotor speed, with longer use correlating with greater impairment. However, although there have been a number of studies into the long-term effects of cannabis of memory and learning, most are small or not sufficiently rigorous. We cannot, therefore, make any clear conclusions on this topic. Verbal memory appears to be the specific type of memory that is particularly vulnerable, but there is no clear evidence of long-term damage.
It may be that the effects of cannabis on cognition depends on various factors. For example, there is some evidence that starting in adolescence may be more likely to produce impairment, some suggestion that it depends on the strain of cannabis (there’s some evidence that cannabidiol can counteract the memory-impairing effects of the psychoactive component THC), a possibility that certain conditions could interact (one study found that MS sufferers who used marijuana for pain relief performed significantly worse on tests of attention, speed of thinking, executive function and visual perception of spatial relationships between objects).
Some research suggests that regular use of Ecstasy (MDMA) may result in long-term memory problems, particularly in the area of prospective memory (remembering things you plan to do). It also appears that even a single dose can affect verbal memory.
There is also some evidence that long-term use may reduce the number of brain cells, especially in the hippocampus. Use of Ecstasy during pregnancy may damage the developing fetus, but only if taken during a particular critical period of pregnancy.
Having said that, a recent study designed to overcome the potential confounding factors in previous studies found little evidence of decreased cognitive performance in ecstasy users, with the exception of poorer strategic self-regulation (which may be a precursor rather than a consequence).
Rat studies have indicated that high doses of amphetamines in adolescence produced significant declines in working memory as adults, and that amphetamines and cocaine can damage the brain’s ability to learn from new experiences. A human twin study also found long-term cocaine or amphetamine abuse affected attention and motor skills. Another human study suggests that cocaine use particularly affects prospective memory — which of course is heavily dependent on executive function.
One important mechanism for these effects involves the neurotransmitter dopamine. Methamphetamine abuse damages neurons that respond to dopamine. And a small imaging study found that people with different variants of the COMT gene (which affects dopamine responsiveness) were differently affected by amphetamine in regards to their performance on tasks involving working memory and executive functioning (this may explain why some people respond better to ADHD drugs).
Prenatal exposure to methamphetamine or cocaine has been found to produce structural abnormalities in the brain. For cocaine, the damage seems to lie mainly in regions regulating attention and memory. Mental retardation, according to one study, is dramatically (nearly five times) higher in cocaine-exposed children. Nevertheless, a review of 32 major studies of school-age children concludes that, with the exception of attention problems, the low IQ and poor academic and language achievement often found in those whose mothers used cocaine during pregnancy is more related to home environment than to the drug exposure.
Among those exposed to methamphetamine, the damage appears to be primarily to the striatum (significantly smaller) and the limbic system (significantly bigger). The striatum is involved in skill learning and habit formation, and motor coordination is likely to be affected.