With the exception of benzodiazepines the evidence for the role of prescribed drugs in road trauma is uncertain. In general, most drugs tend not to be significant risk factors on the road when the drugs are used as prescribed.
Some drugs can cause impairment due to their central nervous system depressant properties, particularly early in treatment before the patient becomes accustomed to the drug, or when the drug is misused.5
Table 1 shows some prescription drugs and their relative risk of causing impairment. The most common examples seen in road trauma are the anticonvulsants and the antidepressants, but their presence does not necessarily mean that they had a contribution to the crash.
In many cases two or more impairing drugs including alcohol are detected. Combinations of drugs increase the opportunity for impairment and the risk of a serious crash.
Table 1 Medicines that may impair driving skills
Drug
|
Risk of causing impairment *
|
Anticonvulsants (such as carbamazepine, gabapentin, phenobarbitone, phenytoin, valproate, vigabatrin)
|
Moderate to high
|
Antihistamines - sedating (such as azatadine, chlorpheniramine, cyproheptadine, diphenhydramine, promethazine, doxylamine, trimeprazine)
|
Moderate to high
|
- less sedating (such as cetirizine, desloratidine, fexofenadine, loratidine)
|
Low to moderate
|
Antipsychotics (such as amisulpride, chlorpromazine, haloperidol, pericyazine, clozapine, olanzapine)
|
Moderate to high
|
Benzodiazepines and related compounds (such as temazepam, nitrazepam, oxazepam, alprazolam, clonazepam, diazepam, zolpidem, zopiclone)
|
Moderate to high
|
Drugs for diabetes
|
Low to moderate
|
Muscle relaxants (such as baclofen, dantrolene, orphenadrine)
|
Moderate
|
Opioid analgesics (such as codeine, buprenorphine, methadone, morphine, oxycodone, pethidine, tramadol)
|
Moderate to high
|
Serotonin, mixed reuptake inhibitors and reversible monoamine oxidase inhibitor antidepressants (such as fluoxetine, sertraline, paroxetine, citalopram, venlafaxine, moclobemide)
|
Low
|
Tricyclic and tetracyclic antidepressants (such as amitriptyline, clomipramine, dothiepin, doxepin, imipramine, trimipramine, mianserin, mirtazapine)
|
Moderate to high
|
Sympathomimetics (such as pseudoephedrine, phenylephedrine)
|
Low to moderate
|
Benzodiazepines
Benzodiazepines are well known to increase the risk of a crash6-7
They are found in about 4% of fatalities4
and 16% of injured drivers taken to hospital.3
In many of these cases benzodiazepines were either abused or used in combination with other impairing substances. When abuse occurs, the drugs may not have been prescribed to the person concerned. The illicit trade in these drugs is significant and they are often obtained by 'doctor shopping'. Medical practitioners do need to be aware of this possibility when prescribing benzodiazepines and the related hypnotics zolpidem and zopiclone. If a hypnotic is needed a shorter-acting drug is preferred. Tolerance to the sedative effects of the longer-acting benzodiazepines used in the treatment of anxiety gradually reduces their adverse impact on driving skills.
Antidepressants
Although the antidepressants are one of the more 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, typified by amitriptyline and dothiepin, than with the less sedating serotonin reuptake inhibitors. 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 blockade of 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 such as hydromorphone, morphine or oxycodone have direct effects on driving behaviour. Cognitive performance is reduced early in treatment, largely due to their sedative effects, but neuro adaptation rapidly sets in. This means that patients on a stable dose of an opioid may not have a higher risk of an accident. This includes patients on buprenorphine and methadone for their opioid dependency, providing the dose has been stabilised after 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.
Drugs for diabetes
Hypoglycaemia can be a significant problem. The drugs themselves have no major effect on skills, but how well they control blood glucose will affect driving performance.