When people talk about impaired driving, they usually mean alcohol. That makes sense. Alcohol is the most consistent factor in fatal wrong-way crashes, and ADOT reports that two out of three wrong-way crashes are caused by impaired drivers.
But "impaired" is a broader word than "drunk." Cannabis, prescription medications, and even some ordinary cold and allergy remedies can affect a driver's ability to read the road and react in time. During the holiday season, when people are tired, sick, celebrating, and traveling all at once, those effects can stack up.
Legal to use, illegal to drive impaired
Arizona voters approved Proposition 207 in 2020, legalizing recreational marijuana for adults. It did not change the rule about driving. A memo from Coconino County prepared after the measure took effect lists driving while impaired to the slightest degree by THC as an offense under Arizona's DUI statute.
Arizona's DUI law, A.R.S. 28-1381, applies to driving under the influence of alcohol, any drug, or any combination when the driver is impaired to the slightest degree. Prescription medications are not exempt simply because they were prescribed.
NHTSA puts it plainly: it is illegal in all 50 states and the District of Columbia to drive impaired by any substance, legal or illegal, and even in states where marijuana laws have changed, it is still illegal to drive under its influence.
What cannabis does to driving
NHTSA summarizes the research this way: marijuana impairs motor skills, lane tracking, and cognitive functions. The agency also cites research showing that THC reduces the ability to multitask, a skill that driving depends on constantly.
Those are exactly the abilities a driver needs at a freeway interchange at night: tracking the lane, reading signs, noticing that something is wrong, and correcting before it becomes a crash.
Drug use among drivers is also more common than many people assume. In NHTSA's 2013–2014 national roadside survey, 20% of surveyed drivers tested positive for potentially impairing drugs. Among weekend nighttime drivers, the share testing positive for marijuana rose from 8.6% in 2007 to 12.6% in 2013–2014.
A later study of trauma centers from October through December 2020 found that 56% of drivers involved in serious and fatal crashes tested positive for at least one drug.
Medications that can affect driving
The U.S. Food and Drug Administration lists many types of medicine that can make driving dangerous, including:
- Sleeping pills
- Prescription anxiety medicines such as benzodiazepines
- Opioids, including some cough suppressants that contain codeine or hydrocodone
- Muscle relaxants
- Some antidepressants
- Some prescription and over-the-counter cold and allergy medicines that contain an antihistamine
- Medicines for motion sickness
- Stimulants and "stay awake" products
The FDA also notes that cannabis and cannabis-derived products, including CBD, could make driving dangerous.
Cold and flu season overlaps with holiday travel. A driver who takes a nighttime cold medicine before a long drive may not think of it as impairment at all.
Practical steps
The FDA's advice for medicines is simple and worth following:
- Talk to your doctor or pharmacist about whether a medicine could affect driving.
- Read the Drug Facts label on over-the-counter products and follow the warnings.
- Take a new medicine for the first time when you will not need to drive, so you know how it affects you.
- Tell your health care provider about everything you take, including vitamins and supplements, since combinations can matter.
For cannabis, the plan should look the same as it does for alcohol: if you are going to use it, decide ahead of time how you are getting home, and do not drive until you are no longer impaired.
Combining substances
Holiday gatherings sometimes involve more than one substance at once: a drink or two, a cold medicine, a sleep aid the night before a trip. Arizona's DUI law covers impairment from any combination of alcohol and drugs, and the safest assumption is that combining them makes things worse, not better.
If you are not sure whether you are safe to drive, you are not. Wait, ride with someone else, or call for a ride.
Why it matters for wrong-way crashes
The reason impairment drives so many wrong-way crashes is not just slower reaction. It is that impaired drivers are less likely to notice they have made a mistake: that the sign is facing the wrong way, that the ramp is curving the wrong direction, that the headlights are coming toward them.
That is true regardless of which substance caused the impairment.
A broader definition of a sober ride
NMWWD is a Phoenix-based nonprofit working to prevent wrong-way crashes. When we encourage people to plan a sober ride home, we mean sober from everything that affects driving, not just alcohol.
If you would like to help us share that message with more Arizona drivers this season, get involved or make a donation.
Sources: NHTSA — Drug-Impaired Driving · U.S. Food and Drug Administration — Some Medicines and Driving Don't Mix · Arizona Revised Statutes — 28-1381, Driving under the influence · Coconino County — Proposition 207 memo · Arizona Department of Transportation — Wrong-Way Drivers