Drug-impaired driving is a crime. It falls under the DUI category because the use of a substance can alter your ability to safely operate a motor vehicle.
The California Office of Traffic Safety notes that drugged driving can happen whether you use legal or illegal drugs. It could happen when you use prescription drugs or over-the-counter medication. Any substance that alters your perception or impacts your reflexes could land you with a DUI citation for drugged driving.
Officers will make stops for drugged driving in the same way they would when suspecting someone is driving under the influence of alcohol. They use clues from your driving to determine if you could be impaired. Swerving, driving slowly for no reason or applying the brake randomly are all examples of clues an officer may see.
Despite having some probable cause to pull you over, the officer still must conduct some tests and gather more evidence that you may be under the influence of something that is impairing your driving. For alcohol, they have a breath test they can use, but nothing like that is available for drugs. They will usually have to take you into custody and run blood tests to determine if you have a substance in your system.
If you use medications or substances, you should be cautious about driving. Always read the warnings on medication. Many will include a note that you should not operate heavy machinery after use, which means you should not drive. You can also talk to your doctor about whether your medications will impact your ability to drive.