In cities like Seattle and Vancouver, the marijuana icon has become almost as common on storefronts as the Starbucks mermaid. But there’s one big difference between the products on offer: A venti latte tastes the same everywhere and provides an identical caffeine rush, while marijuana stores offer the drug’s active ingredients in varying combinations, potencies and formats. There is no consistency in testing, standards or labeling.

This matters because marijuana’s two psychoactive ingredients, tetrohydrocannabinol (THC) and cannabidiol (CBD), have contrasting effects on the brain. “THC makes you feel high,” said Catharine Winstanley, a psychology professor at the University of British Columbia who does research on marijuana, while CBD “is responsible for its analgesic, antiseizure and purported anticancer effects.”

In street marijuana, the THC-to-CBD ratio now tends to be 10 to 1, and it is increasing, a trend occurring even at some marijuana clinics, Dr. Winstanley said. And few people know what effect that has on their brains. A new study by Dr. Winstanley’s group in the Journal of Psychiatry and Neuroscience examines how these two chemicals shape our willingness to face a challenge. Does marijuana make us lazy?

To answer this question, the Winstanley team first tested rats to determine which were hard workers and which were slackers. After being injected with THC, CBD or both, the rats had to choose between a simple task with a measly reward or a demanding one that reaped a bigger payoff.

In the easy version, the animals had a minute—an eternity for a rat—to see that a light was on in a chamber and poke their noses inside. They got a single sugar pellet. In the hard version they only had a fifth of a second to notice the light—something a rat, even a stoned one, should have no problem perceiving—and respond. Their vigilance in the hard choice would earn them two lumps instead of one. Under normal circumstances, the vast majority of rats prefer to work harder for a bigger payoff.