As medical cannabis becomes more common in Kentucky, drivers may wonder how marijuana affects cases of driving under the influence (DUI). The five nanograms per milliliter (ng/mL) tetrahydrocannabinol (THC) level that some jurisdictions use often causes confusion. Kentucky takes a different approach. In 2026, the state does not it as an automatic legal limit for marijuana impairment.
Why THC levels matter
Some states set specific THC concentration limits for drivers. These rules may allow prosecutors to rely heavily on the amount of THC that a blood test detects.
Kentucky however, has not adopted a 5 ng/mL per se limit for marijuana. Instead, the state prohibits a person from driving when a substance impairs that person’s ability to drive. Kentucky law also excludes marijuana from the Schedule I substances that can trigger the state’s separate controlled-substance provision based simply on their presence in a driver’s blood.
This distinction matters. A blood test that shows 5 ng/mL or another amount of THC does not automatically prove that a Kentucky driver experienced impairment.
Why impairment matters
The amount of THC in a person’s blood does not necessarily show how much the substance affected that person’s driving. As a result, prosecutors may need to use several types of evidence to establish impairment.
They may point to the person’s driving, behavior during the traffic stop and the officer’s observations. Prosecutors may also use field sobriety test results, blood test results and other evidence from the stop.
Kentucky’s medical cannabis law does not allow registered patients to drive while impaired. A person may legally use medicinal cannabis under state law, but police can still charge that person with DUI if they believe cannabis impaired the person’s ability to drive.
Why evidence matters
Kentucky law draws an important distinction between THC in the body and actual driving impairment. A particular THC number alone does not establish Kentucky’s marijuana DUI standard.
Because these cases can involve scientific evidence and officer observations, speaking with an attorney may help a person understand how Kentucky law applies to the facts. An attorney can also review blood test results, the traffic stop and other evidence that may affect the case.