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How does combining drugs affect driving?

Mixing drugs can have an unpredictable effect, as it is difficult to predict how the drugs will interact.

Drugs could be mixed when:

2 or more drugs are taken at the same time
a drug is taken when another is already present in the body.

This could include mixing illegal drugs and legal drugs, such as alcohol and medicines (prescribed or over-the-counter).

The risk of having a collision while under the influence of two or more drugs may even be higher than if a person is under the influence of a single drug. For example, if alcohol and cannabis are used together the impairment to driving ability can be much greater, and occur more quickly, than using either one on their own (Mallick et al. 2007).

How is driving affected by combining drugs

If multiple drugs are taken the effects are unpredictable for each person. Factors influencing this may include:

the types of drugs used
how much of each drug is used
when the drugs are taken, in relation to each other
the mind and body of the person using it.

These factors make it difficult to predict exactly, and in what way, mixing drugs may affect a person's ability to drive safely.

As a guide, some of the effects of mixing drugs are outlined below.

Combining drugs with similar effects

Combining drugs with similar effects (such as alcohol and cannabis, alcohol and benzodiazepines, or amphetamines and ecstasy) can increase the effects of each drug. This can place greater strain on your body and organs, and increases the risk of overdose.

Depressant drugs (like alcohol, cannabis, heroin and other opioids, and benzodiazepines) slow down the activity of the central nervous system.

Combining different depressants can multiply the depressant (slowing down) effects on the central nervous system. For example, some of the effects of combining depressant drugs include:

reduced reaction times
reduced concentration
drowsiness
difficulty processing information
difficulty doing more than one thing at a time (e.g. keeping the car within its lane, and in the right direction, while watching the traffic).

Stimulant drugs (like amphetamines, cocaine and ecstasy) speed up the activity of the central nervous system.

Combining stimulants can multiply the stimulant effects on the the central nervous system. This puts a greater stress on the body, particularly the heart and other vital organs. For example, some of the effects of combining stimulant drugs include:

attention difficulties
a tendency to fidget
aggressive and dangerous driving
increased risk taking
over-confidence in driving skills that is not supported by an actual improvement in driving ability.

Hallucinogens (like ketamine, LSD, magic mushrooms, mescaline and PCP) distort a person's perception of reality.

People who have taken them may see or hear things that aren't really there, or what they see may be distorted in some way.

Ecstasy and cannabis can also have some hallucinogenic effects.

The effects of hallucinogens vary, and combining two or more drugs of this type can have unpredictable effects on driving ability.

As a general guide, some of the effects of combining hallucinogenic drugs include:

hallucinations
impaired thinking
blurred vision
reduced coordination.

Combining drugs with different effects

Combining drugs with different effects (such as alcohol with ecstasy, or cocaine with benzodiazepines) appears to cancel out the effects of each. This makes it difficult for a person to judge how much their driving ability has been impaired.

If a person has been drinking alcohol and using amphetamines, they may not feel the depressant effects of the alcohol, as they have been masked by the stimulant effects of the amphetamines. The person may feel capable of driving, but in reality they may be drunk.

Safer driving tips

Mixing drugs and driving is dangerous as it is difficult to predict exactly how drugs may interact with each other and how long the effects will last. If you intend to use drugs, the safest option is not to drive.

Speak to a health professional and read the packaging information or a Consumer Medication Information sheet (CMI) before mixing medications, alcohol or othe drugs.

Tell the prescribing doctor about all of the drugs (including prescribed, over-the-counter and illegal drugs) that you use. This can help them to minimise the risk of unwanted interactions between drugs.

If you are using medicines, it is important to ask your doctor or pharmacist if it is safe to drink alcohol.

Find more information for drivers.