2.2.8 Drugs and driving
Any drug that acts on the central nervous system has the potential to adversely affect driving skills. Central nervous system depressants, for example, may reduce vigilance, increase reaction times and impair decision making in a very similar manner to alcohol. In addition, drugs that affect behaviour may exaggerate adverse behavioural traits and introduce risk-taking behaviours.
Acute impairment due to alcohol or drugs (including illicit, prescription and over-the-counter drugs) is managed through specific road safety legislation that prohibits driving over a certain blood alcohol concentration (BAC) or when impaired by drugs (refer to Appendix 4: Drivers’ legal BAC limits). This includes requirements for using alcohol interlocks for high-risk offenders, the application of which varies between jurisdictions (refer to Appendix 5: Alcohol interlock programs). This is a separate consideration to long-term medical fitness to drive and licensing, thus specific medical requirements are not provided in this publication. Dependency and substance misuse, including chronic misuse of prescription drugs, is a licensing issue and standards are outlined in Part B section 9 Substance misuse (including alcohol, illicit drugs and prescription drug misuse).
Where medication is relevant to the overall assessment of fitness to drive in the management of specific conditions, such as diabetes, epilepsy and psychiatric conditions, this is covered in the respective chapters. Prescribing doctors and dispensing pharmacists do, however, need to be mindful of the potential effects of all prescribed and over-the-counter medicines and to advise patients accordingly. General guidance is provided below.
General guidance for prescription drugs and driving
While many drugs have effects on the central nervous system, most, with the exception of benzodiazepines, tend not to pose a significantly increased crash risk when the drugs are used as prescribed and once the patient is stabilised on the treatment. This may also relate to drivers self-regulating their driving behaviour. When advising patients and considering their general fitness to drive, whether in the short or longer term, health professionals should consider the following:
- the balance between potential impairment due to the drug and the patient’s improvement in health on safe driving ability
- the individual response of the patient – some individuals are more affected than others
- the type of licence held and the nature of the driving task (i.e. commercial vehicle driver assessments should be more stringent)
- the added risks of combining two or more drugs capable of causing impairment, including alcohol
- the added risks of sleep deprivation on fatigue while driving, which is particularly relevant to commercial vehicle drivers
- the potential impact of changing medications or changing dosage
- the cumulative effects of medications
- the presence of other medical conditions that may combine to adversely affect driving ability, and
- other factors that may exacerbate risks such as known history of alcohol or drug misuse.
(refer also PART B section 9 Substance misuse (including alcohol, illicit drugs and prescription drug misuse))
- Benzodiazepines. Benzodiazepines are well known to increase the risk of a crash and are found in about 4 per cent of fatalities and 16 per cent of injured drivers taken to hospital. In many of these cases benzodiazepines were either abused or used in combination with other impairing substances. If a hypnotic is needed, a shorter acting drug is preferred. Tolerance to the sedative effects of the longer acting benzodiazepines used to treat anxiety gradually reduces their adverse impact on driving skills.
- Antidepressants. Although antidepressants are one of the more commonly detected drug groups in fatally injured drivers, this tends to reflect their wide use in the community. The ability to impair is greater with sedating tricyclic antidepressants, such as amitriptyline and dothiepin, than with the less sedating serotonin and mixed reuptake inhibitors such as fluoxetine and sertraline. However, antidepressants can reduce the psychomotor and cognitive impairment caused by depression and return mood towards normal. This can improve driving performance.
- Antipsychotics. This diverse class of drugs can improve performance if substantial psychotic-related cognitive deficits are present. However, most antipsychotics are sedating and have the potential to adversely affect driving skills through blocking central dopaminergic and other receptors. Older drugs such as chlorpromazine are very sedating due to their additional actions on the cholinergic and histamine receptors. Some newer drugs are also sedating, such as clozapine, olanzapine and quetiapine, while others such as aripiprazole, risperidone and ziprasidone are less sedating. Sedation may be a particular problem early in treatment and at higher doses.
- Opioids. There is little direct evidence that opioid analgesics (e.g. hydromorphone, morphine or oxycodone) have direct adverse effects on driving behaviour. Cognitive performance is reduced early in treatment, largely due to their sedative effects, but neuroadaptation is rapidly established. This means that patients on a stable dose of an opioid may not have a higher risk of a crash. This includes patients on buprenorphine and methadone for their opioid dependency, providing the dose has been stabilised over some weeks and they are not abusing other impairing drugs. Driving at night may be a problem due to the persistent miotic effects of these drugs reducing peripheral vision.