How Medications Affect Driving Abilities
Many medications can impair a person's ability to drive safely. This impairment is not limited to illicit drugs; many legal, doctor-prescribed, and over-the-counter (OTC) medicines can have dangerous side effects. The side effects that are particularly risky for drivers include sleepiness or drowsiness, blurred vision, dizziness, slowed movement, and decreased ability to focus or pay attention. These effects can mimic the impairment caused by alcohol, and some studies even compare the level of impairment caused by certain drugs to specific blood alcohol concentration (BAC) levels.
Key Drug Classes That Cause Drowsiness
Antihistamines
Used to treat cold and allergy symptoms, antihistamines are a common culprit for drowsy driving. They are typically divided into two generations based on their sedating effects:
- First-generation antihistamines: These medications, like diphenhydramine (Benadryl), easily cross the blood-brain barrier and cause significant sedation. They can slow reaction time and impair focus, even if the person does not feel overtly sleepy.
- Second-generation antihistamines: These are often marketed as "non-drowsy" because they do not cross the blood-brain barrier as readily. Examples include loratadine (Claritin) and fexofenadine (Allegra). However, some people still experience sedation, particularly with cetirizine (Zyrtec).
Sleep Medications (Hypnotics)
Prescription sleep aids, or hypnotics, are explicitly designed to induce sleep but can have dangerous residual effects that impair driving the next day.
- Benzodiazepine hypnotics: Drugs like temazepam (Restoril) can impair driving performance the morning after a dose, and these effects can persist for many hours depending on the drug's half-life.
- Non-benzodiazepine hypnotics (Z-drugs): Medications such as zolpidem (Ambien), eszopiclone (Lunesta), and zaleplon (Sonata) are popular sleep aids. While some, like zaleplon, are short-acting, others, like zolpidem, carry a significant risk of next-day impairment, especially in women and with higher doses. Rare but serious side effects can include sleep-driving.
Anti-Anxiety Medications (Benzodiazepines)
Prescribed for anxiety and panic disorders, benzodiazepines like alprazolam (Xanax), lorazepam (Ativan), and diazepam (Valium) work by depressing the central nervous system, leading to sleepiness, dizziness, and slowed reaction times. The risk of impaired driving is particularly high when starting treatment or increasing a dose, even for long-term users.
Opioid Pain Relievers
Opioids such as codeine, morphine, and oxycodone are powerful pain relievers that can cause drowsiness and confusion. A study on fatal two-vehicle crashes found that drivers testing positive for prescription opioids had a significantly increased risk of initiating the crash. The impairing effects are most pronounced when a person first starts taking the medication or increases the dosage.
Antidepressants
Some antidepressants can cause drowsiness, especially in the initial stages of treatment. Older tricyclic antidepressants (TCAs) like amitriptyline (Elavil) are well-known for their sedating effects. While newer selective serotonin reuptake inhibitors (SSRIs) and serotonin-norepinephrine reuptake inhibitors (SNRIs) are less sedating, some, like mirtazapine, still pose a risk.
Muscle Relaxants
Prescribed for muscle spasms and pain, muscle relaxants like cyclobenzaprine (Flexeril) and methocarbamol (Robaxin) affect the central nervous system and cause significant sedation. They can impair motor skills and coordination, making driving extremely unsafe. Combining them with alcohol or other depressants drastically increases the risk.
Comparison of Drowsiness Risk by Medication Class
To highlight the varying levels of risk, the table below provides a general comparison. It is important to note that individual reactions can differ, and a person's risk can increase when combining medications or with alcohol.
| Medication Class | Examples | Typical Sedation Risk | Key Concerns for Driving |
|---|---|---|---|
| First-Gen Antihistamines | Diphenhydramine (Benadryl) | High | Significant sedation, slowed reaction time, difficulty concentrating |
| Second-Gen Antihistamines | Cetirizine (Zyrtec), Loratadine (Claritin) | Low to Moderate (varies) | Possible sedation, especially with cetirizine; some individuals are more sensitive |
| Sleep Medications (Hypnotics) | Zolpidem (Ambien), Temazepam (Restoril) | High | Next-day residual sedation, delayed clearance, especially in women |
| Anti-Anxiety Meds (Benzodiazepines) | Alprazolam (Xanax), Lorazepam (Ativan) | High | Central nervous system depression, dizziness, impaired judgment |
| Opioid Pain Relievers | Codeine, Oxycodone | High | Sedation, impaired cognitive and motor skills, particularly when starting use |
| Tricyclic Antidepressants | Amitriptyline (Elavil) | High | Marked sedation, especially at higher doses |
| SSRIs/SNRIs | Fluoxetine (Prozac), Duloxetine (Cymbalta) | Low to Moderate | Drowsiness more common during initial treatment; individual reactions vary |
| Muscle Relaxants | Cyclobenzaprine (Flexeril) | High | Sedation, poor coordination, impaired motor skills |
Managing Drowsiness and Driving Safely
If you take any medication that causes drowsiness, it's essential to follow these safety measures:
- Read the label carefully: Always check the warnings on both prescription and OTC medications. Labels often state, "Caution: May cause drowsiness," or warn against operating heavy machinery, which includes driving.
- Talk to your doctor or pharmacist: Discuss potential side effects and how they may affect your driving, especially when starting a new medication or changing a dose. They can advise on safer alternatives or dosing schedules.
- Know your body's reaction: Avoid driving until you understand how a new medication affects you. Your initial reaction may be different from a long-term one.
- Plan alternatives: If your medication is highly sedating, arrange for a ride-share service, public transport, or a trusted driver, especially at night.
- Avoid mixing: Never combine sedating medications with alcohol, as this can dramatically amplify the impairing effects.
Conclusion
Driving is a complex task that requires full attention, coordination, and swift reactions. Medications from common OTC allergy relievers to powerful prescription drugs can compromise these abilities, causing dangerous drowsiness and other forms of impairment. Recognizing that what drugs often cause drowsiness while driving extends far beyond illicit substances is the first step toward safety. Always consult a healthcare professional about potential side effects, read warning labels, and err on the side of caution. Your awareness can prevent a life-threatening accident. To learn more about drug safety, visit the U.S. Food and Drug Administration's consumer updates page.
