Surprising Effects of Drugs on Driving Safety: Amazing!

Driving  requires doing many things at the same time. It also requires dealing with the unexpected. It’s important to know the effects of drugs on driving safety. They can either reduce or enhance driving ability.

Alphabetical Listing.

    • Amphetamine
    • Caffeine
    • Cannabis (Marijuana)
    • Carisoprodol and Meprobamate
    • Cocaine
    • Dextromethorphan
    • Diazepam
    • Diphenhydramine
    • Ethanol (Beverage Alcohol)
    • Gamma-hydroxybutyrate (GHB) and its precursors of GBL and 1,4-BD
    • Ketamine
    • Lysergic acid diethylamide (LSD)
    • Nicotine
    • Opioids (morphine and heroin)
    • Phencyclidine (PCP)
    • Toluene
    • Zolpidem (and zaleplon, zopiclone)
    • Effects of Drugs on Driving: Effects of Specific Drugs

Amphetamine (Methethamphetamine and Ecstacy)

effects of drugs on drivingAmphetamine is a powerful central system stimulant. Some of its effects can improve driving skills. It can increase alertness, improve concentration, increase energy, and improve some thinking skills. This includes the ability to divide attention.

But other effects of amphetamine can reduce safe driving. It can cause tunnel vision, poor impulse control, agitation, irritability, and paranoia. It can lead to aggressiveness, greater self-confidence, grandiosity, and strong feelings of power and superiority.

Improved psychomotor skills occur among sleep-deprived people on amphetamine. But the effects on thinking abilities are inconsistent. Some truck drivers use it to increase their energy and alertness when tired.

Methethamphetamine is also a central nervous system stimulant. Its effects can include greater alertness, faster reaction time, poor coordination, paranoia, poor impulse control, aggression, delusions and hallucinations.

Ecstasy produces effects similar to those of amphetamine. It is a less powerful stimulant. But it can reduce vision, cause rigid muscles, tremors, reduced attention, and less ability to divide attention.

Predicting the effects of ecstasy on driving performance is hard. It can cause greater driving speed and changes in speed. But it doesn’t degrade all driving tasks. It improves some. The effects of drugs on driving is not simple.


drugs on drivingCaffeine is a stimulant. It reduces sleepiness and increases alertness temporarily. People use it to satisfy addiction to it, for its taste, and for its stimulative effects. When drivers are drowsy, caffeine improves their driving. It also increases alertness and reaction time after drinking alcohol. But it doesn’t eliminate alcohol impairment.

Alertness in driving is important. The National Highway Traffic Safety Administration estimates that about 100,000 crashes occur in the U.S. annually from drowsy driving.

Cannabis (Marijuana)

drugs on driving
Marijuana leaf

Cannabis is a naturally occurring drug. It contains substances called cannabinoids. These include tetrahydrocannabinol or THC. THC apparently causes most of the psychoactive effects of cannabis.

Marijuana is the most common illicit drug in the world. People usually smoke it but some consume it orally. After alcohol, it is the most common psychoactive substance that drivers use.

At typical recreational use levels, there are many driving related effects of cannabis. They include drowsiness, inability to concentrate, impaired memory, problems in thinking, distorted perceptions of time and space. Also disorientation, paranoia, feelings of panic, reduced inhibitions, and reduced coordination.

Various factors influence the specific effects of any given THC  blood concentration. These include the experience of the user and the individual vulnerability of the user to psychoactive reactions. Also important are  the expectations of the user as to the effects of marijuana and the setting of its use.

Most of the effects of marijuana on behavior disappear within three to five hours after the beginning of use. Butit effects some skills for as long as 24 hours. The more complex and difficult the skill, the more likely marijuana will reduce ability.

Selective attention is the ability to disregard irrelevant information. It is an important driving skill. Long-term users have reduced selective attention, even after they quit using the drug. Similarly, increased frequency of marijuana use impairs information processing speed. What are the effects of using of marijuana along with drinking alcohol? The use of these drugs on driving may cause much greater impairment than either substance alone.

Carisoprodol and Meprobamate

Carisoprodol is a central nervous system depressant that is prescribed to relax muscles. Meprobamate is also a central nervous system depressant. It reduces anxiety.

Either of these drugs reduces visual functioning, slow thinking and cause confusion. Also disorients, reduces comprehension, and causes sluggish movements, tremor, and loss of coordination.

The effects of carisoprodol begin within half an hour. They continue for as long as six hours. The effects of meprobamate continue much longer.

It appears that single doses of carisoprodol do not effect thinking ability or psychomotor performance. However, single doses of meprobamate can impair the ability to divide attention, slow reflexes, and impair coordination.

The effects of driving under the influence of either carisoprodol or meprobamate include sleepiness, inattentiveness and slow responses. Also poor perception, slower reaction time, confusion, disorientation and slurred speech. In addition, it causes lack of coordination and difficulty standing, walking or exiting vehicles. The use of other central system depressants can increase impairment. For example, drinking alcohol increases the sedation and confusion that carisoprodol can cause. It also increases the sleepiness, confusion, incoherence and disorientation that meprobamate can cause. The effect of both drugs on driving is to increase risk of crashes.


drugs on drivingCocaine is a powerful central nervous system stimulant. It increases alertness, reduces fatigue, improves mental focus and enhances mental clarity. It also improves the performance of some simple tasks, creates euphoria, and creates feelings of well-being and strength.

At higher doses, cocaine can cause mental confusion, disorientation, fear, paranoia, aggression, delusions, hallucinations, and lead to anti-social behavior.

Some researchers have found no effect of cocaine on thinking or psychomotor performance. However, others have found cocaine to enhance attention, learning, and psychomotor performance. Cocaine reduces the negative effects on performance of sleep deprivation or alcohol intoxication.


The over-the-counter drug dextromethorphan reduces coughs from minor throat and bronchial irritation. Recreationally it raises mood, increases perceptual awareness and creates euphoria. It also can create dream-like experiences and a sense of separation of the mind from the body.

Negative effects from taking recommended doses of dextromethorphan are rare. However, slight drowsiness and dizziness can occur. Taking higher doses for recreational reasons can lead to disorientation, confusion, reduced memory and impaired judgment. It can also lead to psychomotor impairment, distorted perception of time, and hallucinations of both sight and sound.

Combining dextromethorphan with promethazine can cause great drowsiness or impair the mental or physical abilities needed to drive safely.


Diazepam is a central nervous system depressant. It is widely useful in medical practice. Recreationally it’s a sedative and it also enhances the effects of alcohol or opioids. Cocaine users sometimes use it to increase the level at which seizures occur. Heroin users often use it to increase the effects of heroin. And both cocaine and heroin users often use it to reduce withdrawal symptoms.

A single low dose of medically prescribed diazepam can significant impair driving related skills. The largest effects occur about two hours after dosing and can last for over four hours. Impairments can include reduced vigilance, poor thinking ability, reduced memory and reduced ability to divide attention. Also decreased attention, reduced reaction time, decreased eye-hand coordination, slower braking time, and increased lane crossing. Drinking even small amounts of alcohol can increase impairments.

Recreational use typically involves higher doses. These can case slowed reflexes, slurred speech, disorientation, tremor, blurred or double vision, hallucinations, and amnesia. Some of these impairments can last for days.


Diphenhydramine is a central nervous system depressant. Medicinally it relieves allergy symptoms, relieves coughs, promotes sleep, and prevents motion sickness.

Other central nervous system depressants increase the effects of diphenhydramine. These include alcohol, sedatives, tranquilizers, hypnotics, diazepam, and monoamine oxidase inhibitors (MAOIs). Alcohol increases drowsiness and decreases motor skills.

Diphenhydramine reduces alertness, causes drowsiness, impairs concentration, reduces vigilance and slows reaction time. It reduces working memory, reduces divided attention, impairs the estimation of time, decreases learning ability, and reduces psychomotor performance.

In on-the-road driving tests, diphenhydramine impairs ability to avoid letting the vehicle weave or drift. It reduces or slows reaction time, and reduces overall driving performance. A single dose of diphenhydramine for medical purposes can dramatically impair psychomotor skills for four hours. It reduces driving performance more than the consumption of alcohol at a level over the maximum legal limit.

Ethanol (Beverage Alcohol)

drugs on drivingEthanol is a central nervous system depressant. That is, it slows the action of our nervous system. This in turn slows heartbeat, breathing, and other bodily functions.

The level of ethanol in the bloodstream is blood alcohol concentration or BAC. Measuring it requires analyzing a sample of a person’s blood. However, testing samples of exhaled breath only estimates BAC. Unfortunately, this method is lacks a high degree of reliability.

BAC is expressed in terms of grams of absolute alcohol per deciliter of blood (g/dl). For example, .08 g/di. The “g/dl” usually replaced by “percent” or “%.” But this is not actually a percentage. That’s because the .08 represents a measure of weight in a measure of volume.

After drinking ends, BAC will rise for a period of time before it begins to drop. It declines at a constant rate of about .015 of BAC per hour.

     Alcohol is Sexist

Take males and females of the same height and weight. Give them the same amount of alcohol to drink in the same length of time. They will have different BACs. Specifically, the male’s BAC will be substantially lower. This is because females process alcohol differently. Females also have a higher proportion of body fat than men. Fat does not absorb alcohol. Therefore, it concentrates at a higher level in the bloodstream of females.

Impairment of different driving skills occurs at different BAC levels. For example, impairment of alertness usually occurs at a lower BAC level than reaction time and vision. There can also be large differences in the degree of impairment between people who have the same BAC. For example, people who drink large amounts of alcohol over a period of years developed a tolerance for the substance. As a result, it takes more alcohol to have the same effects.

A drinker’s expectations can also effect the degree of impairment. For example, impairment of some skills can occur if people falsely believe that they have been drinking alcohol. On the other hand, their impairment can be reduced if they falsely believe that they have not been drinking alcohol.


drugs on drivingNicotine is a naturally occurring drug that is ordinarily a central nerve system stimulant. However, it changes from a stimulant to a sedative as the dosage increases.

When smokers want a stimulating effect, they generally take short quick puffs. This causes a low-level of nicotine in the bloodstream. When they want a sedative effect, they generally inhale deeply. This causes a higher level of nicotine in the bloodstream.

As with caffeine, nicotine can have a significantly beneficial effect on athletic performance. Nicotine can help reduce traffic crashes among tired or sleep-deprived drivers. It does this by increasing alertness and energy.

Opioids (Morphine and Heroin)

Both morphine and heroin are central nervous system depressants. Morphine is a naturally occurring substance obtained from a poppy plant. Morphine produces heroin.

The effects of morphine or heroin depend on the dose, the route of administration, and tolerance. Both drugs can cause similar effects. These include euphoria, drowsiness, mental clouding, inability to concentrate, distractibility, lethargy, apathy, tremors, slower reaction time, and reduced consciousness. The effects of these drugs on driving is to increase risk of crashes.

The effects of morphine can last for up to six hours. It can cause sedation and psychomotor impairment for up to four hours. The effects of heroin can last up to five hours.

Phencyclidine (PCP)

Phencyclidine or PCP is a synthetic drug. Veterinarians use it as an anesthetic or tranquilizer. Recreational users get it from illegal producers or illegally from veterinary sources. They use it as a hallucinogen and psychedelic drug.

A single dose of PCP can create severe impairment of driving skills. The effects include blurred vision, reduced memory, confused thinking disorientation, poor concentration, hallucinations and paranoia. Also feelings of power and invincibility, combativeness, numbness in extremities, reduced eye-hand coordination. Users may have distorted sensory perceptions including distorted perceptions of space.

The effects of PCP typically occur within an hour after taking the drug. A reduced or slowed reaction time can occur for up to 14 hours.


Toluene is a central nervous system depressant. It occurs naturally in the tolu tree and in crude oil. Oil refining, making coke from coal, making styrene produce it.

Toluene is a solvent in paints, glues, and nail polish remover. Printers and tanners also use it.

Toluene causes drowsiness, reduced alertness, confusion, impaired color vision and reduced ability to concentrate. Also memory loss, slowed reaction time, delusions, hallucinations, and distorted perception of time and space.

There appears to be no clear association between the level of toluene in the bloodstream and the degree of impairment. Toluene use is common among adolescents. That’s because it is readily available in many products that are inexpensive and legal to purchase. However, some states prohibit the sale of some such products to minors.

Zolpidem (and Zaleplon, Zopiclone)

Zolpidem, zaleplon and zopiclone are central nervous system depressants. Doctors prescribe them on a short-term basis for insomnia.

Each of these drugs can cause drowsiness, slow reflexes, problems thinking clearly, double vision and confusion. Also difficulty concentrating, impaired memory, slow reflexes, poor coordination, and hallucinations. Some of these impairments can last up to eight hours.

Higher doses of zaleplon can cause driving-related impairments for up to ten hours. Higher doses of zopiclone can cause severe negative effects on tests of driving performance for up to ten hours. Thus, the effects of these drugs on driving is negative.

Conclusion: Effects of Drugs on Driving Safety

Research suggests that drugged driving is a much more widespread and serious problem than generally recognized. There is a common lack of understanding about the powerful effects of drugs on driving ability.

Police routinely check drivers for evidence of alcohol in their systems. But they almost never check for either legal or illicit drugs. So drugged drivers are much likely to escape arrest than those under the influence of alcohol. Yet the effects of drugs on driving can be equal to or greater than that of alcohol.

On the other hand,  the surprising fact is that some drugs contribute to traffic safety and help save lives.

Mothers Against Drunk Driving

We have made great strides in reducing drunken driving. Much of the credit goes to Mothers Against Drunk Driving (MADD). For over four decades MADD has raised awareness of drunken driving. It has promoted laws against it, and worked tirelessly to reduce the crime.

But MADD has historically opposed any consideration of drugged driving. It has seen the issue of drugged driving as competing for public attention, support, and funding. This has been a serious disservice to the public and its welfare. It’s important to understand the effects of drugs on driving.

Remember. A person killed by a drugged driver is just as dead as one killed by a drunken driver. In either case, it’s the effects of drugs on driving.


Adapted from David J. Hanson, The Effects of Substances on Driving. In: P. Miller (Ed.) Principles of Addiction. NY: Acad Press/Elsevier, vol. 1. Chapter 39, pp. 371-379. That chapter looks at the effects of drugs on driving in more detail.