This Is How a Bunch of Drugs Affect Your Brain's Ability to Drive

The science behind cannabis, hallucinogens, opioids, and stimulants.

by Matilda Whitworth
27 March 2018, 12:30am

Illustration by Ben Thomson

This article is supported by the Transport Accident Commission.

I work in a busy inner-city emergency department and there isn’t a shift that goes by where I’m not looking after someone who’s affected by drugs. Even though these patients can increase our workload, I’m always grateful that they’re not out on the streets—causing harm to themselves or others.

Driving under the influence of drugs is a pretty serious road safety issue. According to Victorian data, approximately 18 percent of drivers killed on the state’s roads tested positive to cannabis, and in the past five years 11 percent of drivers killed had ecstasy, speed, or crystal meth in their system.

We look at the effect a bunch of different drugs have on your brain, and its ability to drive.


As you probably know, cannabis is the most commonly used illicit drug in Australia. According to the most recent National Drug Strategy Household Survey, around 10 percent of people over the age of fourteen have used it in the last twelve months. So it's not surprising that cannabis is the drug most often detected by police when screening drivers.

When THC (the active ingredient of cannabis) binds to cannabinoid receptors in the brain, it causes both depressant and hallucinogenic effects. Just like when alcohol, a depressant, is consumed, if you've smoked weed you may drive too slowly, find it hard to stay awake, or may not have quick enough response times to take evasive action when needed.

In a 2015 study researchers showed that elevated blood THC concentrations led to increased weaving within a lane. They also found that when combined with alcohol, drivers wove significantly more, even if the THC and alcohol were below the levels that would normally cause cognitive impairment.

Probably best to stay at home watching Adventure Time and steal your housemate's food rather than drive to the shops for more snacks.


When the inventor of LSD, Albert Hoffman, first tried his new creation, he wrote this about his experience: "The faces of those around me appeared as grotesque coloured masks… marked motoric unrest, alternating with paralysis… heavy feeling in the head, limbs and entire body, as if they were filled with lead."

Here lies the danger of driving under the influence of hallucinogens like LSD and magic mushrooms: your sense of reality can become completely distorted. You may swerve to dodge something that isn’t there, not see a person standing on the road or, if you do see that person, have your reflexes and coordination so blunted that there’s nothing you can do to avoid an accident.

LSD alters the way your brain works by mimicking the chemical messenger serotonin. While serotonin is well known for making us feel good, it has many varied roles in the body, one of which may include maintaining your grip on reality. “We think serotonin helps keep a handle on perception and actually stops us from hallucinating,” Clare Stanford, a psychopharmacologist at University College London, told The Guardian.

LSD has a higher affinity for the 5-HT2 family of serotonin receptors than serotonin itself, meaning that when LSD binds it prevents serotonin from sending messages in the brain. Recent research published in the journal Cell has shown that LSD gets stuck in these receptors, resulting in hallucinations that can last for hours.

Despite being derived from amphetamines (which affect different neurotransmitters), ecstasy also works on serotonin. By inhibiting its transport and stimulating its release, ecstasy causes a serotonin flood in certain brain regions.


Opioids can be really useful drugs, particularly for people experiencing intense pain. Codeine, oxycodone, buprenorphine, fentanyl, and morphine are often used in emergency departments for this reason. But despite their utility, this family of drugs (which also includes heroin) has the potential to make driving very dangerous.

Opioids work by binding to opioid receptors, of which there are four different kinds: mu, kappa, delta and (the lesser known sorority) the nociception receptor. It is the mu receptors in the central nervous system that are responsible for the pain relief effects of opioids, but also the effects of decreased breathing (‘respiratory depression’), euphoria, and sedation.

While opiates are most likely to cause death by respiratory depression, it is their effect on cognition that can cause issues when driving. Driving on opiates can lead to reduced reaction times, poor concentration, and drowsiness (including falling asleep at the wheel), as well as trouble processing information and multitasking.

A recent study of 434 New Zealanders published in the journal Accident; Analysis and Prevention showed that when questioned participants felt that both opiates and hallucinogens were the illegal drugs that produced the highest level of driving impairment. While this is reassuring, the study also found that only 42.5 percent of people taking strong painkillers made the decision not to drive, which could indicate a knowledge gap between the negative impacts of legal versus illegal opiates.


Amphetamine and methamphetamine work by releasing dopamine and noradrenaline from neurons in the brain. Cocaine on the other hand binds to and inhibits the transporters that bring these chemicals back into the neurons, leaving them to continue their effects. When these chemicals flood the brain, a person may experience increased movement, euphoria, insomnia, and potentially psychosis. Not behaviours you want behind a wheel.

“Stimulants are likely to cause people to be more impulsive, have impaired decision-making, and also possibly be more aggressive, all of which would be likely to impede their driving ability”, Professor Andrew J Lawrence, the Florey Neuroscience Institute's Head of Behavioural Neuroscience, tells VICE.

So what can people do to reduce their risk of causing drug-related road accidents? While the medical professional in me would say not to use recreational drugs at all, the pragmatist in me would recommend that if you decide to use drugs—illegal or not—avoid driving altogether.

This article is supported by the Transport Accident Commission. You can find out more info about driving and drugs here.