While a person in Virginia may lawfully be prescribed medical marijuana for certain conditions, that doesn't mean that the use of the drug couldn't land them in legal hot water. Specifically, not unlike alcohol, police who suspect a motorist is driving while high on marijuana may issue the motorist a citation for driving while intoxicated. This is due to the belief that certain drugs can impair a motorist's ability to safely operate an automobile.
There is a catch with trying to arrest someone for drugged driving, however. If a person is suspected of drunk driving, police can perform a breath test to measure the motorist's blood alcohol concentration. If the motorist's BAC is 0.08 percent or higher, that motorist may be deemed drunk per se and will receive a DUI.
However, such roadside chemical tests for marijuana use aren't as reliable. There isn't a scientifically agreed-upon limit on how much tetrahydrocannabinol (THC) can be in a person's system before that person is too impaired to drive. Marijuana can remain in a person's system for weeks -- long after the initial "high" has worn off. Thus, the mere presence of marijuana in a person's system does not indicate that the motorist is intoxicated per se.
Nevertheless, police will still rely on observations and field sobriety tests to determine if a person is driving under the influence of drugs. This means that a person could be charged with DWI if the police believe that person was high on marijuana while driving. When this happens, a strong criminal defense strategy is needed to show that the motorist was sober when issued the citation and therefore the charges against them should be dismissed.