Driving while high impairs the same skills you need to stay safe on the road

Driving while high—whether on cannabis, prescription medications, or other drugs—slows your reaction time, distorts your depth perception, and weakens your judgment about speed and distance. Your brain processes information more slowly, your hands respond later to what you see, and you're more likely to misjudge how fast you're going or how close you are to other cars. These aren't small changes. A half-second delay in braking at highway speed means traveling an extra 40 feet before your car slows down.

The law treats drug-impaired driving the same way it treats alcohol impairment. You can be arrested for driving under the influence (DUI) or driving while impaired (DWI)—the exact term depends on your state—even if you're below the legal alcohol limit or haven't had any alcohol at all. The charge applies to any substance that affects your ability to drive safely, including prescription medications, over-the-counter drugs, and illegal drugs.

Key Takeaways

  • Cannabis, prescription opioids, sedating antihistamines, and many other drugs slow reaction time and impair judgment behind the wheel, even at doses that feel mild.
  • You can be arrested for DUI or DWI based on impaired driving alone—police do not need a breathalyzer result or a positive drug test to make the arrest.
  • Conviction carries license suspension, fines, mandatory education programs, and possible jail time; a second offense within ten years typically doubles the penalties.
  • Some states use roadside impairment tests (walk-and-turn, one-leg stand, horizontal gaze nystagmus) to establish probable cause, and blood or urine tests can confirm drug presence but not impairment level.
  • Prescription medications carry the same legal risk as illegal drugs if they impair driving, even at prescribed doses.

How different drugs affect driving ability

Cannabis slows reaction time and impairs your ability to track moving objects and judge distance. The effect peaks within the first hour but can linger for hours afterward. THC concentration in your blood does not reliably predict impairment the way blood alcohol does—a heavy user may show high THC levels while feeling alert, while an occasional user may be significantly impaired at lower levels. This mismatch is why many states struggle with roadside testing for cannabis.

Prescription opioids (codeine, hydrocodone, oxycodone) cause drowsiness, slow reaction time, and reduce attention. Even at prescribed doses, they can impair driving. The risk is highest when you first start taking them or when your dose increases. Mixing opioids with alcohol or other sedating drugs multiplies the danger.

Benzodiazepines (Xanax, Valium, Ativan) and other anti-anxiety medications slow your central nervous system. They reduce alertness and increase reaction time, and the effect can last well into the next day, especially with longer-acting versions. Combining them with alcohol or opioids is particularly dangerous.

Antihistamines (diphenhydramine in Benadryl, chlorpheniramine) cause drowsiness and slow reaction time. Newer non-drowsy formulas (cetirizine, loratadine) have less impact, but older sedating versions significantly impair driving. Many people underestimate how much they affect alertness because they're available over the counter.

Stimulants (prescription amphetamines, cocaine, methamphetamine) can cause overconfidence, aggressive driving, and poor judgment about speed and distance. You may feel alert but drive recklessly. The crash afterward brings fatigue and impaired attention.

What police look for and how they test for impairment

Police do not need a positive drug test to arrest you for impaired driving. If an officer observes signs of impairment—swerving, slow reaction at a traffic light, difficulty following instructions—they can make an arrest based on those observations alone. The arrest comes first; the test comes after.

At the roadside, an officer typically asks you to perform standardized field sobriety tests: the walk-and-turn (walk nine steps in a line, turn, walk back), the one-leg stand (stand on one leg for 30 seconds), and the horizontal gaze nystagmus test (follow a moving object with your eyes). These tests measure balance, coordination, and attention—all affected by drugs. Poor performance gives the officer probable cause to arrest you and take you to a station or hospital for a chemical test.

Chemical tests measure drug presence in your blood or urine. A positive result shows that a drug is in your system, but it does not prove you were impaired at the time you were driving. THC, for example, can remain in your blood for days or weeks after use, long after impairment has worn off. Some states have set legal limits for certain drugs (similar to the 0.08% blood alcohol limit), but these limits are controversial because they do not reliably predict impairment.

Criminal penalties for drug-impaired driving

A first DUI or DWI conviction typically results in license suspension (usually 3 to 12 months), fines ranging from several hundred to several thousand dollars, and mandatory enrollment in a DUI education or treatment program. Some states also require an ignition interlock device—a breathalyzer built into your car that prevents the engine from starting if it detects alcohol. You may face jail time, though first offenses often result in probation instead.

A second offense within a set period (usually 5 to 10 years, depending on your state) carries harsher penalties: longer license suspension, higher fines, longer mandatory programs, and a greater likelihood of jail time. A third or subsequent offense may be charged as a felony, resulting in years of incarceration and permanent criminal record.

Beyond the criminal case, a DUI or DWI conviction affects your insurance rates (often doubling or tripling them for years), your employment (especially if your job involves driving or requires a clean record), and your ability to rent housing or obtain certain professional licenses. The conviction stays on your record permanently in most states.

Prescription medications and the law

Taking a prescription medication exactly as prescribed does not protect you from a DUI charge if the medication impairs your driving. The law focuses on impairment, not on whether you were following a doctor's instructions. If your medication causes drowsiness, slows reaction time, or affects judgment, you can be arrested for driving while impaired.

Before you start a new prescription, ask your doctor or pharmacist whether it affects driving ability. Many medications carry a warning label about operating machinery or driving. If a medication does impair driving, ask whether a different dose, timing, or alternative medication might work better. Some people can take sedating medications at night and drive safely during the day; others need to avoid driving entirely while taking them.

If you're charged with DUI while taking a prescribed medication, your prescription and doctor's statement about the dose may help your defense, but they do not may provide acquittal. The prosecution will argue that you should have known the medication impaired you and should not have driven.

What to do if you're stopped and suspected of impairment

If an officer pulls you over and suspects impairment, you have the right to remain silent and the right to an attorney. You are not required to perform roadside sobriety tests, though refusing them may be used against you in court (and in some states, refusal itself carries penalties). You are required to provide your license, registration, and proof of insurance.

You are required to submit to a chemical test (blood, breath, or urine) if the officer arrests you and requests one. Refusing a chemical test after arrest carries serious penalties—automatic license suspension and possible criminal charges—separate from any DUI conviction. The penalties for refusal are often as severe as the penalties for a positive test.

Do not argue with the officer, do not admit to drug use, and do not consent to a search of your vehicle without a warrant. Tell the officer you want to speak to an attorney before answering questions. Contact a DUI attorney as soon as possible; many offer free initial consultations and can advise you on your specific situation and state laws.

Alternatives to driving while impaired

If you use any substance that impairs driving—whether prescription, over-the-counter, or recreational—plan ahead. Call a taxi or rideshare service, use public transportation, ask a sober friend for a ride, or stay where you are until the impairment wears off. The cost of a ride is far less than the cost of a DUI conviction, and you avoid risking your life and others' lives.

If you regularly take medications that impair driving, talk to your doctor about timing doses to minimize impact on your driving schedule, switching to a non-impairing alternative, or adjusting your work or activity schedule to avoid driving when impaired. Some people find that taking a sedating medication at night allows them to drive safely during the day.

Frequently Asked Questions

Can I be arrested for DUI if I smoked cannabis yesterday but I'm not high now?

You can be arrested if an officer observes signs of impairment—swerving, slow reaction, difficulty following instructions—regardless of when you last used cannabis. A positive drug test does not prove current impairment, but it can support the officer's observations. Your defense would focus on proving you were not impaired at the time of driving, not on when you last used the drug.

What's the difference between DUI and DWI?

The terms vary by state. Some states use DUI (Driving Under the Influence) for all impaired driving. Others use DWI (Driving While Impaired) or DWI (Driving While Intoxicated) for drug or alcohol impairment. A few states distinguish between them based on substance type or blood alcohol level. Check your state's laws for the exact definitions and penalties.

If I take my medication as prescribed, can I still be convicted of DUI?

Yes. The law does not care whether you followed your prescription. If the medication impairs your driving ability and an officer observes signs of impairment, you can be arrested and convicted. Your prescription may help your defense, but it does not prevent arrest or may provide acquittal.

Do I have to take a roadside sobriety test?

No, you can refuse roadside tests (walk-and-turn, one-leg stand, eye test). Refusing may be used against you in court, but you have the legal right to decline. You are required to provide your license and registration, and you must submit to a chemical test (blood, breath, or urine) if arrested and the officer requests one.

How long does cannabis impairment last?

Peak impairment typically occurs within the first hour after smoking or ingesting cannabis, but effects can linger for several hours. The exact duration depends on the dose, the person's tolerance, and the method of use. Edibles take longer to kick in but can impair driving for 4 to 8 hours or longer. When in doubt, do not drive.