Taking medication when you’re sick can make driving significantly more dangerous, sometimes worse than driving after drinking alcohol. Both the illness itself and the drugs you take to manage symptoms can impair your reaction time, focus, and coordination. In a study published in the Annals of Internal Medicine, a standard dose of a common over-the-counter antihistamine (diphenhydramine, the active ingredient in Benadryl) impaired driving performance more than being at the legal alcohol limit.
This isn’t just a safety recommendation. Driving while impaired by any substance, including over-the-counter cold medicine, can result in a DUI arrest under state laws. About 56% of drivers involved in serious injury and fatal crashes tested positive for at least one drug, according to NHTSA data from 2020.
Being Sick Already Impairs Your Driving
Before you even open a medicine cabinet, illness alone degrades your ability to drive safely. Fever, fatigue, dehydration, and the general brain fog that comes with a cold or flu all reduce your reaction time, narrow your attention, and slow your decision-making. These effects mirror many of the same impairments caused by drowsy or distracted driving.
The NHTSA’s review of medical conditions and driving performance documents how even moderate symptoms create real risk behind the wheel. Conditions that cause fatigue, poor concentration, dizziness, or blurred vision are all flagged as driver-impairing. A bad cold or flu can produce every one of those symptoms. When you layer medication side effects on top of that baseline impairment, the combined effect is greater than either one alone.
Which Medications Impair Driving Most
The biggest offenders are medications containing first-generation antihistamines. These are found in many nighttime cold and flu formulas, allergy pills, and sleep aids. Diphenhydramine is the most common. It enters the brain easily, causing drowsiness, slowed reaction time, blurred vision, and mild confusion. In the Iowa driving simulator study, participants taking a standard dose of diphenhydramine had worse lane-keeping and slower response times than participants who had consumed alcohol. They drifted across the center line more often and reacted more slowly to a vehicle blocking their path.
The FDA warns that antihistamines can impair your focus and reaction time even when you don’t feel drowsy. That’s a critical point: you may feel alert enough to drive while your actual cognitive and motor performance tells a different story.
Other common medications that affect driving include:
- Cough suppressants containing dextromethorphan, which can cause dizziness and drowsiness
- Anti-nausea medications, many of which are sedating antihistamines themselves
- Pain relievers that contain codeine or other opioids, sometimes found in prescription cough syrups
- Muscle relaxants prescribed for body aches, which cause significant drowsiness
- Sedating anxiety or sleep medications, where impairment can persist well into the next day
How Long to Wait Before Driving
Wait times depend on what you’ve taken. Research on sedating medications provides some benchmarks: after a dose of a strong sedative like lorazepam, impairment can last 24 hours. Milder sedating drugs may clear enough to drive after 5 to 7 hours, though individual responses vary.
For common cold and flu medications containing first-generation antihistamines, the sedating effects typically peak within 1 to 3 hours and can linger for 4 to 6 hours. Federal guidelines for commercial truck drivers are stricter: they must wait at least 12 hours after taking a sedating medication before getting behind the wheel. That 12-hour rule is a reasonable benchmark for anyone, not just professional drivers.
If you’ve taken a nighttime cold formula before bed, you may still be impaired the next morning. These medications are designed to help you sleep, and their sedating ingredients don’t always wear off by the time your alarm goes off.
Choosing Medications That Are Safer for Driving
Not all cold and allergy medications carry the same risk. Second-generation antihistamines like loratadine (Claritin), cetirizine (Zyrtec), and fexofenadine (Allegra) are far less likely to impair driving. In the same Iowa driving simulator study, fexofenadine produced no meaningful impairment compared to a placebo, while diphenhydramine performed worse than alcohol.
If you need to drive while managing cold or allergy symptoms, look for products labeled “non-drowsy” and check the active ingredients. Avoid anything containing diphenhydramine, doxylamine, or chlorpheniramine. These are the first-generation antihistamines most likely to affect your driving. Plain acetaminophen or ibuprofen for fever and aches won’t impair your ability to drive, though they don’t address the underlying fatigue and brain fog from being sick.
Be careful with multi-symptom formulas. A single product marketed for cold and flu might combine a non-sedating decongestant with a sedating antihistamine or cough suppressant. Always read the label for drowsiness warnings.
The Legal Risk Is Real
Many people assume DUI laws only apply to alcohol or illegal drugs. They don’t. Every state has laws making it illegal to drive while impaired by any substance, and that includes prescription medications and over-the-counter drugs you bought at a pharmacy. If an officer pulls you over and you’re exhibiting signs of impairment (swerving, slow reactions, confusion), the fact that you took a legal medication is not a defense.
NHTSA’s 2013-2014 National Roadside Survey found that 20% of drivers on the road tested positive for at least one potentially impairing drug. Many of those were likely legal medications taken by people who didn’t realize the risk.
Practical Steps When You’re Sick and Need to Get Somewhere
The safest option is to not drive at all when you’re noticeably ill. If you have a fever, feel foggy, or can barely keep your eyes open, your driving is already compromised before any medication enters the picture. Use a rideshare, ask someone for a ride, or reschedule if you can.
If driving is unavoidable, choose non-drowsy medications and take them well before you need to be on the road so you can gauge how they affect you. Stick to single-symptom products rather than combination formulas, so you know exactly what you’re taking. Avoid nighttime formulas entirely if you’ll be driving within 12 hours. And pay attention to how you actually feel: if your eyes are heavy, your thinking is sluggish, or you’re having trouble concentrating on routine tasks at home, those same impairments will follow you into the car.
Commercial drivers face an even higher bar. Federal regulations require that drivers with common illnesses like colds, flu, or bronchitis be relieved from duty until treatment is complete and they’ve waited at least 12 hours after their last dose of any sedating medication. A medical examiner can disqualify a driver at any point if they believe a medication or medical condition poses a safety risk.