Prescription Medication and Drug Driving: Could Your Medicine Put You Over the Limit?

Not every drug-driving case involves illegal drugs. The offence under section 5A of the Road Traffic Act 1988 also captures a number of prescription and over-the-counter medicines, and drivers are sometimes surprised to learn that a medicine taken exactly as directed can, in principle, put them over a specified limit.

Medicines that fall within the limits

The specified limits cover several drugs that are commonly prescribed, including:

• Benzodiazepines such as diazepam, temazepam, lorazepam, oxazepam and clonazepam
• Opioids such as morphine (and medicines that produce it in the body)
• Methadone
• Amphetamine, which is prescribed for certain conditions

For these medicinal drugs the limits are generally set at higher, risk-based levels than for drugs like cocaine or cannabis (broadly intended to sit above normal therapeutic concentrations), but a driver can still exceed them, for example where doses are high, where more than one medicine is involved, or where individual factors affect how the drug is processed.

The statutory ‘medical defence’

Recognising that patients must be able to drive, the law provides a specific statutory defence. In broad terms it applies where:

• the drug was lawfully prescribed, supplied or sold for a medical or dental purpose;
• it was taken in accordance with the directions of the prescriber or the accompanying instructions; and
• the driver was not in fact impaired.

The defence is not automatic. It has to be raised and supported, and the prosecution can seek to rebut it, for example by arguing that the medicine was not taken as directed, or that the concentration found is not consistent with proper therapeutic use.

Where the toxicology matters

This is often where a forensic toxicologist becomes central to the case. Relevant questions can include:

• Is the blood concentration consistent with the prescribed dose and dosing schedule?
• Could the level be explained by a legitimate therapeutic regimen rather than misuse?
• How might other medicines, or timing, have affected the result?
• Do the analytical results themselves stand up to scrutiny?

How ATXpert can assist

An independent report can interpret a driver’s toxicology results against their prescription and dosing history, and set out whether the findings are consistent with taking the medicine as directed. That evidence can be pivotal in supporting the statutory medical defence, or in testing whether the prosecution’s interpretation is sound.

If you are advising a client charged with drug driving in relation to prescribed medication, an early expert assessment can clarify whether the science supports the defence.