Chronic Effects
Over the long term, things get a little worse. It’s important to point out that in epidemiological studies, it can be very difficult to tease out whether cause and effect is actually at play, or whether there's something else going on. But the authors have gone to great lengths to separate causation from correlation, combing the data for studies that point strongly to cause and effect. Here’s what they found:
Marijuana can be addictive. But only for some people. About 10% of all users seem to develop dependence syndrome, and for those who start in adolescence, the number is more like 1 in 6. Withdrawal syndrome is also a real phenomenon, with depression, anxiety, insomnia, and appetite disturbance being the main symptoms, which can often be severe enough to have an effect on daily life.
Marijuana use is linked to adverse cognitive effects. In particular, the drug is linked to reduced learning, memory, and attention. It hasn’t been entirely clear whether these effects persist after a person stops using the drug, but there’s some evidence that it does. One study found a reduction in IQ of 8 points in long-time users, the greatest decline being in people who'd started using as teenagers and continued daily into adulthood. For people who began in adulthood and eventually stopped using, a reduction in IQ was not seen a year later.
Marijuana may change brain structure and function. There’s been an ongoing debate about whether marijuana actually changes the brain, but recent evidence has suggested that it is linked to changes in the hippocampus, amygdala, and prefrontal cortex. It’s unclear, however, how long these effects last, whether they're linked to behavioral changes, and whether they reverse after a person stops using the drug.
Regular use is linked to an increased risk of psychotic symptoms. That marijuana is linked to increased psychotic symptoms (e.g., delusions, hallucinations, disordered thinking) is fairly clear. But again, it’s been a chicken-and-egg problem, since it’s hard to show whether causation is at play, and which way the connection goes. However, it’s likely that the relationship actually goes both ways: Marijuana may lead to psychotic symptoms, and early psychotic symptoms may increase the likelihood that a person will smoke marijuana (particularly if there’s a family history of psychotic disorders).
Marijuana is linked to lower educational attainment. When pot smoking begins in adolescence, people tend to go less far in school – but again, a causal relationship hasn’t been demonstrated.
Marijuana may (or may not be) be a gateway drug. Regular teenage marijuana users are more likely to use other drugs in the future – but again, researchers don't know whether the link is causal.
Marijuana is probably – but modestly – linked to schizophrenia. The study found that marijuana is connected to a doubled risk of a schizophrenia diagnosis in the future. Many previous studies have suggested this connection, but, as always, showing causality is hard. The new study cites a number of well-executed studies that suggest a causal relationship between marijuana and schizophrenia. The authors estimate that marijuana use may double the risk of schizophrenia from 7 in 1000 non-users to 14 in 1000 marijuana users. On the upside, they point out that users who quit using the drug after a first psychotic episode have fewer psychotic symptoms and better social functioning moving forward, compared to people who have a psychotic episode but continue using.
Marijuana may be linked to testicular cancer. Its connection to other forms of cancer is not very consistent, but there's some evidence of an increased risk of testicular cancer in long-term marijuana users.
Regular users may have cardiopulmonary issues. Regular marijuana users have a higher risk of developing chronic bronchitis. Marijuana “probably” increases the risk of heart attack in middle age, but it’s hard to know for sure, since many users also smoke cigarettes.
