All drugs have side effects, including medical marijuana; the question is whether or not these side effects are manageable or outweighed by the benefits of the drug. For example, ibuprofen can cause liver damage, but used as directed, it's very rarely going to cause harm. When it comes to cannabis, however, there is still a prevailing stigma that the "medical" part is just a euphemism and that the drug can cause significant cognitive impairment.
Yet a recent study in the journal CNS Drugs found that medical marijuana patients who use the drug as prescribed do not experience meaningful negative cognitive effects after doing so. The researchers tested 40 patients by giving them a spectrum of marijuana products, including oils and flower, and then performing a battery of tests to measure cognitive performance. The authors found "no evidence for impaired cognitive function," concluding, "these findings suggest that prescribed medical cannabis may have minimal acute impact on cognitive function among patients with chronic health conditions, although larger and controlled trials are needed."
In a press release regarding the results, the study's lead author — Dr. Thomas Arkell of the Swinburne University of Technology’s Centre for Human Psychopharmacology — explained that “we already know that non-medical cannabis can impact memory and attention. However, our findings show that patients prescribed medical cannabis by a doctor don’t experience the same effects." He added that over time patients may develop tolerance to the negative cognitive effects of delta-9-tetrahydrocannabinol (THC), the primary psychoactive component of cannabis.
This is not the only recent study to show medical benefits to marijuana. In September a study in the journal Frontiers in Psychiatry found that because "the endocannabinoid system plays a key role" in autism spectrum disorders, marijuana can have positive effects on autistic people.
