Some 56% of people who use medical cannabis for chronic pain say they have driven under the influence of the drug within two hours of using it, according to a new study from the University of Michigan.

Of the nearly 800 medical marijuana users surveyed in 2014 and 2015, 51% admitted to driving while "a little high" and 21% reported driving while "very high."

Study authors say people under the influence of marijuana exhibit slower reaction time and coordination. In a risky roadway situation they are more likely to be involved in a collision because the ability to make decisions and implement them is greatly affected.

According to the authors, people should wait several hours to operate a vehicle after using cannabis. The safest strategy is to forgo driving at all on the day of using marijuana.

There are nearly 270,000 medical marijuana users in Michigan, which is second only to California for the highest number of medical marijuana patients in a single state.

There are 2.1 million medical marijuana users across 33 states where it is legal, reports CBS News. However, there is no reliable roadside test at the moment to determine with confidence if a driver is impaired by cannabis.