Drugs of nearly every category increase the risk of a car crash, but the size of that risk varies dramatically depending on the substance. In a study of trauma centers during late 2020, 56% of drivers involved in serious injury and fatal crashes tested positive for at least one drug. That figure includes prescription medications, illicit substances, and even over-the-counter products most people wouldn’t think twice about taking before driving.
Prescription Opioids
Opioid painkillers carry some of the highest crash risks of any drug class studied. A large analysis comparing each person’s driving record during periods on and off opioid prescriptions found striking results. During periods when patients were taking standard doses, their crash risk was nearly four times higher than during their off-treatment periods. At moderate doses, crash risk jumped to about 5.5 times higher. These numbers held up even when looking only at people on long-term opioid therapy lasting 90 days or more, where moderate-dose users still had roughly five times the crash risk of their own off-treatment baseline.
Opioids impair driving through multiple pathways. They slow reaction time, cause drowsiness, and reduce the ability to divide attention between tasks like checking mirrors, maintaining lane position, and responding to unexpected hazards. Because many people take opioids daily for chronic pain, the exposure window is long and consistent, which partly explains why the crash numbers are so high.
Cannabis and THC
Cannabis increases crash risk, but the picture is more complicated than with opioids. In a prospective study published in the journal Addiction, drivers with THC blood levels at or above 2 nanograms per milliliter had about 1.7 times the unadjusted crash risk of THC-negative drivers. At higher concentrations (5 ng/ml or above), the estimated risk rose to 1.74 times higher, though this result did not reach statistical significance due to the small number of drivers at those levels.
One important wrinkle: once researchers adjusted for alcohol use and driver demographics in a separate large fatality study, marijuana’s contribution to fatal crash risk was no longer statistically significant on its own. This doesn’t mean cannabis is safe to drive on. It means that in real-world crashes, cannabis use heavily overlaps with alcohol use, making it difficult to isolate marijuana’s independent effect. THC impairs tracking ability, slows reaction time, and reduces the capacity to handle multiple driving tasks at once. But the dose-response relationship is harder to pin down than with alcohol, partly because THC blood levels drop rapidly after use while the cognitive effects linger longer.
Stimulants: Cocaine and Amphetamines
Stimulant drugs create a different kind of danger behind the wheel. Rather than sedation, the primary risks are overconfidence, aggression, and risk-taking behavior. A national survey found that lifetime stimulant users were three times more likely to report driving under the influence of drugs and 3.4 times more likely to report speeding while impaired, compared to non-users. They were also about twice as likely to have had their license revoked.
For people meeting the clinical criteria for stimulant use disorder, the numbers were far worse. They were roughly 5.5 times more likely to drive under the influence and nearly four times more likely to speed while impaired. These effects held after adjusting for demographics and general aggression levels, meaning the drugs themselves, not just the personality traits of users, drive the risky behavior. Stimulants can also cause erratic decision-making, tunnel vision, and eventually fatigue during the crash phase as the drug wears off.
Benzodiazepines and Sleep Medications
Anti-anxiety medications and sleep aids are widely prescribed and consistently linked to crash risk. Benzodiazepine anti-anxiety drugs were associated with a 42% increase in the odds of being responsible for a crash in a large responsibility-based study. Sleep-inducing Z-drugs (a newer class of prescription sleep aids) showed a significant association with crash responsibility across multiple time periods studied.
These drugs impair many of the same functions alcohol does: coordination, reaction time, judgment, and the ability to process visual information quickly. The impairment can persist into the morning after a nighttime dose, which is why next-day drowsiness from sleep medications is a well-recognized driving hazard.
Over-the-Counter Antihistamines
Perhaps the most surprising crash risk comes from a product available in any pharmacy aisle. In a controlled driving simulator study, a single standard dose of diphenhydramine (the active ingredient in Benadryl and many nighttime cold medicines) impaired driving performance more than alcohol at a blood alcohol concentration of 0.10%, which is above the legal limit in every U.S. state. Participants on diphenhydramine had worse lane-keeping ability and poorer coherence (the ability to maintain steady control of the vehicle) than participants who were legally drunk.
Newer antihistamines like fexofenadine (Allegra) did not produce this effect. The difference comes down to whether the drug crosses into the brain. Older, “first-generation” antihistamines do, causing significant sedation. If you take any allergy or sleep product containing diphenhydramine, doxylamine, or chlorpheniramine, you should treat it like a sedating prescription drug when it comes to driving.
Combining Drugs With Alcohol
A common assumption is that mixing drugs and alcohol creates a multiplied crash risk far beyond either substance alone. The actual data is more nuanced. A study examining the interaction between drugs and alcohol in fatal crashes found no statistically significant multiplicative effect. In other words, the crash risk from being drug-positive was roughly the same whether the driver was sober or had been drinking. The risks add together rather than multiplying.
That said, when researchers separated marijuana from other drugs, an important distinction appeared. Drivers positive for drugs other than marijuana had nearly double the fatal crash risk of drug-negative drivers regardless of their alcohol level. Marijuana’s independent contribution disappeared after adjusting for alcohol and demographics. This suggests that for many drug categories, the impairment risk is real and substantial on its own, and adding alcohol simply layers one impairment on top of another.
Why Drug Testing Doesn’t Equal Impairment
One reason drug-impaired driving is harder to address than drunk driving is the gap between detection and actual impairment. Blood remains the most reliable testing method because it correlates most closely with what’s happening in the brain. But even blood tests have significant limitations. THC concentrations in blood drop rapidly after use, while cognitive impairment takes much longer to fade. Chronic cannabis users can test positive for THC metabolites long after any impairment has passed.
This creates a real problem for both law enforcement and research. A positive drug test after a crash doesn’t prove the driver was impaired at the moment of impact. It’s one reason crash risk estimates for cannabis tend to be lower and less consistent than for opioids or benzodiazepines, where the relationship between blood levels and impairment is more straightforward. For the average person, the practical takeaway is simple: any substance that affects your brain, whether it’s a prescription, an illegal drug, or a pink allergy pill from the drugstore, affects your ability to drive safely.