Drugs – whether prescription, over-the-counter or illegal drugs – can impair necessary driving skills including vision, reaction time, judgment, hearing, and simultaneous task processing/accomplishment. Driving requires other cognitive skills, such as information processing and psychomotor skills, which may also be impaired by the use of drugs. When drugs are mixed with alcohol, the results can be devastating.

  • Marijuana
    Marijuana has been linked to the impairment of the ability to drive a vehicle. Concentration is affected and there is difficulty in perceiving time and distance, which can lead to the following: bad judgment, impaired reaction time, poor speed control, an inability to accurately read signs, drowsiness, and distraction. When marijuana is combined with alcohol it creates greater impairment in areas such as reaction time and coordination.

When combined with sedatives and opiates, it can cause an increase in anxiety and even hallucinations, along with an increase in heart rate and blood pressure when used with amphetamines. On the other hand, effects are somewhat unpredictable when marijuana is combined with stimulants, such as nicotine, caffeine, amphetamines, and cocaine.

  • Cocaine
    Cocaine may successfully mask fatigue; however, high dosages impair judgment and interfere with the ability of the driver to concentrate. Coordination and vision are impaired. There is an increase in impulsive behaviors with tendencies to take more risks and create confusion within the user. A person using cocaine maintains the illusion of being alert and stimulated, although physical reactions are impaired.

Additive effects are noted when cocaine is combined with over-the-counter products, such as diet pills or antihistamines. Cocaine taken with psychotropic drugs, especially antidepressants, can be extremely detrimental.  A person who has extremely high blood pressure and uses cocaine may suffer from a stroke or heart attack.Some users combine cocaine with alcohol and sedatives to cushion the “crash” or feeling of depression and agitation that sometimes occurs as the effects of cocaine wear off. Further research indicates that additive and antagonistic effects can be produced when cocaine is mixed with alcohol.

If cocaine is used in high doses, as in the case of overdose, alcohol will probably have an additive effect on the symptoms that eventually contribute to death.

When cocaine is injected in combination with heroin, sometimes called “speedballing,” there is an increased risk of toxicity, overdose, and death.

  • Tranquilizers 
    The use of tranquilizers produces drowsiness, a lack of coordination, altered perceptions, memory impairment, poor control of speech, and slower reaction time. Effects on driving include poor tracking, difficulty in maintaining lane position, and neglecting roadside instructions.Some people in methadone treatment programs use benzodiazepines to enhance the effects of methadone. When tranquilizers are combined with alcohol or other central nervous system depressants, synergistic effects may be produced, which may be fatal. Alcohol increases the absorption of benzodiazepines, slows their break down in the liver and can cause cardiovascular and respiratory depression. People who take stimulants sometimes take tranquilizers to off set agitation and sleepiness.
  • Opiates 
    Opiates can cause drowsiness, mental confusion, and visual impairment even at lower, moderate doses. A driver may have difficulty keeping the vehicle in the correct lane and may make errors in judgment.

Alcohol greatly increases the present effects of opiates and can lead to respiratory arrest.
A person injecting heroin mixed with cocaine or methamphetamines, known as “speedballing,” produces a stimulant effect. The listed drug combinations increase the risk of toxicity, overdose, and death.

  • Amphetamines 
    The use of amphetamines can interfere with concentration, impair vision, and increase the driver’s tendencies to take risks. Amphetamines should never be taken with a class of antidepressants known as MAO inhibitors, because of potential hypertensive crisis.Amphetamine users sometimes use marijuana and depressant drugs in order to avoid the adverse side effects of the “crash,” therefore creating multiple drug dependencies.

Drug combinations (called poly-drug use) may cause one of three reactions: additive, synergistic or antagonistic.

  • Additive Effects occur when drug combinations produce an effect that is like simple addition, such as the equation: 1 + 1 = 2.
  • Synergistic Effects occur when drug combinations produce an effect that is greater than the sum of the effects of the two drugs, such as the equation: 1 + 1 = 3.
  • Antagonistic Effects occur when a drug combination produces an effect that is less than the sum of the effects of the drugs acting alone, such as the equation: 1 + 1 = 1 or 1 + 1 = 0. (information provided by www.padui.org )