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:
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.1
How is driving affected by combining drugs
The effects of any drug (including multiple drugs) vary from person to person. How combining drugs affects a person depends on many things including their size, weight and health, also whether the person is used to taking it. The effects of combining drugs, as with any drug, also depend on the amount taken.
There is no safe level of drug use. Use of any drug always carries some risk – even medications can produce unwanted side effects. It is important to be careful when taking any type of drug.
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.
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:
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:
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.
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:
Combining drugs with different effects
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 other drugs.
Tell your 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.
1. Mallick, J., Johnston, J., Goren, N. & Kennedy, V. (2007). Drugs and Driving in Australia: a survey of community attitudes, experience and understanding [PDF:660KB]. Melbourne: Alcohol and Drug Foundation.
Last updated: 8 June 2016