TULSA - Oklahomans approved medical marijuana last year. While it’s now legal to use, law enforcement said it has not affected the driving laws.

Tulsa Police Officer Jon Grafton is a drug recognition expert.

He can tell which category of drugs a person has used, including marijuana.

"We don't have to have a blood test but the blood test is more definitive just like an alcohol concentration," Grafton said. "I can articulate that someone is under the influence of alcohol but I can't say absolutely he is at a .08."

Grafton said there is a way to tell how much marijuana is in a person's system, but regardless of the amount, there are too many variables to test whether a person is impaired.

"The variables are the potency of the marijuana itself, the amount of THC contained in it, the frequency with which the person uses it," said Grafton.

Kourtney Heard works as the supervisor in the Toxicology Unit for the Oklahoma State Bureau of Investigations.

They run tests for officers to see which substances are in a person's body.

Heard said testing for alcohol is much different than marijuana because alcohol metabolizes.

"When you consume marijuana it actually goes to the fatty parts of your body and therefore it can stay in your system longer. you can't say whether or not when they consumed it, if it was from recent use or past use."

Grafton said even if have a medical marijuana license, you can still be charged with a DUI.

"We could prosecute against somebody that has a measurable quantity of THC in their blood that has been in a wreck that we've obtained a blood sample on," Grafton said. "Even if we didn't show any signs of impairment."

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