Alberta researchers are trying to figure out how long you need to abstain from weed before getting behind the wheel.

The effects of alcohol on driving are easily measured but roadside testing for marijuana is still in the development phase.

With the federal government planning to legalize cannabis by July, researchers in Edmonton are aiming to study the effects of marijuana on users.

"So how much can you consume? Could you consume a gram in an hour and then wait an hour and drive? We don't know the answer to that," said Dr. Scot Purdon from the Department of Psychiatry at the University of Alberta.

Purdon is leading the study that will examine the effects of marijuana on users cognitive functions such as fine motor skills, attention span, distractibility and verbal learning.

He said he hopes they'll "be able to offer very clear guidance on how long you should be abstinent before the effects have cleared sufficient to operate a motor vehicle."

There is a lack of research on how THC, the active ingredient in marijuana, affects the brain, he said.

The effects of the drug can vary widely from person to person.

"We don't know is how long it takes to return to normal for you," said Purdon. "Do you have all of your memory capacity back, all your motor skills back within an hour or two hours of last use? Or does it take a week? Could it take even a month?"

He said the best evidence right now shows some verbal learning impairments associated with cannabis seem to persist for up to seven days after use.

"And that's for heavy use of a fairly intense product." he said.

Dr. Scot Purdon is leading the investigation into the effects of weed on cognitive functioning.

Purdon said he and his co-investigators — Dr. Cameron Wild from the University of Alberta and Dr. Alexander Penney at MacEwan University — are also examining how the duration and frequency of use, and the age of the user, may contribute to impairment levels.

The study will focus on young adults who describe themselves as heavy users.

Currently, there isn't enough evidence to help medical professionals advise young people of a safe minimum age to use marijuana, he said. "We don't have a whole lot of data to back up our statements that way."

The research could also have implications for students and people working in the trades, or in jobs that require the retention of verbal information, Purdon said.

"Of course it will diminish productivity in whatever job you do — if the effects are residual, if they last," he said. "Again, we don't really know for sure. And that's unfortunate."

The researchers began recruiting study participants three months ago. Researchers still need more than 80 heavy pot users to participate in the study, which is already underway in a lab at MacEwan.

Participants spend about two hours answering questions around mood, history and substance abuse.

Purdon expects to be finished in the next six to nine months but said that will depend on recruitment of participants.

Those interested in participating can call 780-342-5294.

The project is funded by a $35,000 competitive grant sponsored by the the U of A and the University of Calgary. It is administered through the Strategic Clinical Network connected to Alberta Health Services.