Although cannabis use is a controversial topic on both sides of the argument, there is one fact that both can agree on: cannabis use is on the rise.

The National Institute of Drug Abuse reported there were 19.8 million users in 2013, and marijuana is the most commonly used illicit drug in the country.

With the rise of cannabis use recreationally, as it is becoming more legal nationwide, and being used medicinally for diseases and conditions, new research is being conducted that explores the side effects of long-term use.

Since little is known about long-term use, researchers at Lancaster University took the next step and found that cannabis or cannabis-based drugs, when used long-term, can impair memory.

This new report provides implications of cannabis for users, specifically those who are using the drug long-term.

As cannabis continues to become readily available, it allows for long-term use with varying potencies in states that have made recreational use legal, posing the question of whether potency also plays a role in impairment caused by long-term use.

“The potential relevance of potency, particularly with regard to high THC content of new strains of cannabis (e.g., skunk) versus past cannabis strains with lower THC levels, and the potential of cannabis use to induce impairments, is vastly under researched,” Neil Dawson, lead researcher on the study from Lancaster University, said in an email to Laboratory Equipment. “However, there is some evidence that recreational use of high potency cannabis strains can be more detrimental to mental health and the brain than lower potency strains. Even more worrying is the increasing recreational use of synthetic cannabinoid compounds, such as Spice, the long-term effects of which have not been characterized.”

Dawson noted that synthetic cannabis drugs that are more potent have been suggested in recent data to have pronounced negative effects—more so than negative effects associated with traditional cannabis use.

Memory Impairments Revealed in Mice

Researchers studied the effects of long-term cannabis use in mice by exposing them to cannabinoid drug WIN 55,212-2. When the mice were exposed long-term, receiving 22 exposures to the cannabinoid over a 30 day period, they had significant memory impairments.

“The effects we see happen over a relatively short time frame. It’s interesting that these effects arise over this time scale,” Dawson noted. “A big remaining question is whether the effects become even more pronounced as the period of exposure increases. I expect they would, but it remains to be tested. This is a particularly important question given that many people are exposed to cannabis or cannabinoid-based treatments for very prolonged periods of time (e.g., months to years).”

The researchers also discovered that key regions in the brains of the mice, viewed through brain imaging, were impaired. These regions play a role in learning and memory.

Furthermore, long-term use in the mice revealed that the regions disrupted by long-term cannabis use lessened communication between these areas in the brain.

Even with the implications of long-term cannabis use on parts of the brain, cannabis-based therapies continue to be very effective in treating chronic diseases and conditions that improve quality of life.

“Importantly, our work clearly shows that prolonged cannabinoid intake, when not used for medical reasons, does have a negative impact in brain function and memory,” Anna Sebastiao, lead research at the University of Lisbon, said.

But with many people in legalized states using cannabis recreationally and without limitations, this research is crucial to understanding the risks of developing mental health issues or memory problems from long-term use.

Particularly, the research is central for healthy individuals who are using cannabis long-term.

“It is important to understand that the same medicine may re-establish equilibrium under certain disease conditions, such as epilepsy and MS (multiple sclerosis), but could cause marked imbalances in healthy individuals,” Sebastiao concluded. “As for all medicines, cannabinoid-based therapies have not only beneficial disease-related actions, but also negative side effects.”

Recently, there has been attention on people who suffer from Attention Deficit Hyperactivity Disorder (ADHD) using cannabis medicinally when typical ADHD drug treatments do not work.

Having ADHD, associated with short-term memory problems, and using cannabis long-term, also associated with causing memory impairments in mice, brings into question whether combining the two brings more harm than good.

“There may be some benefit in the use of some cannabinoid compounds to treat ADHD, and some recent studies support this idea,” Dawson said, when asked if long-term cannabis use would negatively impact ADHD side effects. “However, we need more thorough and extensive trials to assess both the potential positive and negative effects of cannabinoid treatment in ADHD. We also need more basic research to see if there is any way to counter the negative effects, if these are present in people with ADHD or other patient populations (e.g. epilepsy, MS), who find cannabinoid drug treatment beneficial,” he concluded.

The research was published in the Journal of Neurochemistry.