Steve Dipaola / Reuters

The Scientific Reports study focuses on the effects of taking drugs on a day-to-day basis, rather than what could happen in the long term. It's clear that marijuana is incredibly safe in this regard: Someone who injects heroin, a drug included in the study, could possibly die of an overdose from just one use. By contrast, someone who smokes pot once may not experience any effects at all.

But scientists say more research must be done on marijuana's potential long-term impact on people who smoke or ingest it regularly.

"There is evidence that it relates to some changes in the brain that may impair memory, and that that is particularly the case if it's used regularly among people who are younger," Leonard said. "We don't really know as well about people who are older. Regular frequent use does seem to be associated with intellectual impairments."