A few months ago, I provided a brief summary of some recent peer-reviewed research showing the serious negative health effects of marijuana use.  Since this time, some new and notable studies have been released that further add to the substantial body of scientific evidence mitigating against the legalization of this drug.

In the journal JAMA (Journal of the American Medical Association) Internal Medicine, a group of international researchers from leading universities “found past exposure to marijuana use to be significantly associated with worse verbal memory in middle age,” leading them to recommend that “with recent changes in legislation and the potential for increasing marijuana use in the United States, continuing to warn potential users about the possible harm from exposure to marijuana seems reasonable.”

In the same issue of JAMA Internal Medicine, two researchers from the University of Queensland and Kings College London summarized what is known about long-term marijuana use and cognitive impairment in middle age, reaching the following conclusions:

Decriminalization, medical dispensaries with marijuana for those with a physician's note, and legalization of marijuana in several states have resulted in increased availability and more relaxed views toward marijuana use. However, to our knowledge, relatively little research has been done on the risks of long-term marijuana use ...

Case-control studies have generally found poorer verbal learning, memory, and attention in those who regularly use marijuana than in controls. The size of these differences usually has been related to the duration and frequency of marijuana use and, hence, to the cumulative dose of tetrahydrocannabinol received.

A New Zealand birth cohort study also found that those with heavy marijuana use over several decades had poorer cognitive performance than did those without such use ... Detailed analyses of the New Zealand study point to persistent use of marijuana as the most plausible explanation for the decline in IQ ...

Functional neuroimaging studies provide supportive evidence that those with heavy marijuana use have impaired cognitive functioning. These studies have generally shown reduced activity in those with long-term marijuana use in brain regions involved in memory and attention, as well as structural changes in the hippocampus, prefrontal cortex, and cerebellum ...

There are also other good reasons why we should discourage early and regular marijuana use by adolescents and young adults. This pattern of marijuana use increases the risks of developing dependence, which is associated with an increased risk of other adverse psychosocial outcomes in adolescence and young adulthood. These outcomes include leaving school early, experiencing psychotic symptoms, receiving a diagnosis of schizophrenia and bipolar disorder, using other illicit drugs, and developing depression and anxiety disorders.

In the American Journal of Obstetrics & Gynecology, researchers from the Baylor College of Medicine found that “[m]arijuana use [during pregnancy] was associated with a significantly increased risk of maternal asthma, as well as preeclampsia. Infants born to mothers reporting marijuana exposure were smaller than their peers with smaller head circumference and lower birthweight at delivery.”

In a review article from the Journal of Cardiovascular Pharmacology and Therapeutics, a team that included researchers from the Marshfield Clinic, the Huntington Medical Research Institute, and the University of Southern California reached some troubling results:

Despite the surgeon general warning about marijuana use several decades ago, and strong evidence for deleterious effects on the cardiovascular system, marijuana use remains common both for medical treatment and as a recreational substance. Evidence suggests that marijuana use can serve as a trigger for acute coronary syndromes and that marijuana-related vascular complications are associated with elevated mortality. As such, before more states legalize marijuana for recreational use, it would be prudent to fully evaluate the safety of the drug.

Rather than moving ahead with further legalization efforts, the science clearly shows that current marijuana legalization frameworks should be rolled back and eliminated, and that any new proposals should be shelved indefinitely until we have a complete understanding as to the long-term health impacts of pot use among the general population.