Many of us have popped a cold and flu tablet or an allergy pill and then got behind the wheel of our car – unaware we might be breaking the law.

Such medications can impair your ability to drive, even if you don’t realise it – and road authorities are warning that people must not drive if they are affected by any substance that is banned on the road.

“It is an offence to drive if you are under the influence of drugs, including medicines – and penalties apply,” a Transport for NSW spokesperson said.

“If you might be affected, don’t take the risk – arrange another form of transport.”

Medicines and driving: When is it illegal?

In Australia, some painkillers, hay fever tablets, over-the-counter medicines – or any drugs that cause behavioural changes – could lead to a user being charged with driving under the influence of a drug or alcohol.

Anyone supervising a learner driver under the same conditions faces the same potential penalties.

Drowsiness is one of the most common side effects to watch out for. You can also be deemed unfit to drive if the medicine causes changes in your concentration, vision, mood, co-ordination and reaction times.

In NSW, road authorities are taking this law one step further, amending the definition of “drug” in the Road Safety Act to cover new or emerging substances.

This means drivers in NSW suspected of being under the influence of a new type of illicit drug or prescription medicine might also be charged – and increases the likelihood of a successful prosecution.

How are the medicines detected?

Roadside saliva tests are set up to detect only cannabis, speed and ecstasy, and not common over-the-counter medicines or prescription drugs.

That means these tests will not pick up cold and flu tablets, or medicines for asthma or sinus problems, such as Sudafed.

But roadside police can ask drivers to submit to sobriety assessments if they think they are under the influence of any substance. These tests involve the officer’s own observations about how a person has been driving, their manner, condition and appearance.

Failing this test means you will be arrested and taken to hospital for blood and urine testing. This will detect a wider range of medicines and substances.

Drug driving laws criminalise good drivers, lawyers say

Barrister Greg Barns, a spokesman for the Australian Lawyers Alliance, said Australia’s drug-driving laws were flawed and inherently unfair.

“It may be a person who has taken prescription drugs who is driving perfectly safely. Or it may a person who is on drugs who shouldn’t be driving,” he said.

“It’s not based on science, and that’s the problem I have with drug-driving laws.”

“You might be a person who has been taking a drug for 10 years and know, based on experience, that you are going to be fine,” he said.

“It’s really woolly, it allows for far too much discretion.”

Earlier this year, NSW Greens MP David Shoebridge called for mobile saliva testing to be extended to include all drugs, including medicines.

“Would you care if the drug they took was legal or not? I’d say the majority of people don’t care, it’s about how that drug affects your ability to drive,” he told news.com.au in January.

Dr Barns, who believes it won’t be long until other states follow NSW on the road safety changes, said policy makers must go back to the start.

“It’s the most ridiculous extension of a flawed law,” he said.

“Legal drugs and illicit drugs all need to be treated in the same way as alcohol. There needs to be a measurement beyond which it’s not safe to drive.”

What are the penalties if you are charged?

Penalties vary by state, and include fines, demerit points, loss of licence, community programs and jail time.

In NSW, you can be fined up to $3300 if convicted of DUI and disqualified from driving for three years.

In Queensland, first-time offenders can cop a fine of up to $3413; that doubles if you are caught again within five years.

In Victoria, being caught with drugs in your system will mean you lose your licence for at least six months and will have to take a 12-hour drug-driver program before you’re back on the road.

In South Australia, the fine for a first-time DUI is up to $1200.

In Western Australia, the penalty for a first offence ranges from $900-$2500, with a minimum 10-month licence disqualification.

How do I know if my medicine is safe to use on the road?

It’s difficult to know how an individual will react to a particular medicine — this will depend on the type of medicine, your tolerance, the time between driving and taking a dose, and if you have any other medicines or alcohol in your system.

If you’re unsure:

• Ask your doctor or pharmacist about side effects;
• Follow the instructions on the medicine’s warning label;
• Don’t get behind the wheel straight after taking a strong medicine — you might find that any side effects will settle over time;
• Stop driving, but don’t stop taking your medicine, if you experience side effects such as drowsiness. Talk to your doctor as soon as possible about switching to another medicine;
• Don’t take more than your prescribed dose and don’t mix alcohol with medicines;
• Arrange someone to pick you up instead, or catch a taxi.