Roadside drug testing does not account for impairment levels, like alcohol testing does.(ABC North Coast: Gemma Sapwell )

New research has shown for the first time how long cannabis users are likely to be impaired and when it may be safe for them to drive.

The findings, researchers and advocates say, strengthen the case for changes to drug-driving laws in much of Australia.

Researchers from the Lambert Initiative for Cannabinoid Therapeutics at the University of Sydney discovered users were impaired for between three and 10 hours after taking moderate to high doses of the intoxicating component of cannabis, tetrahydrocannabinol (THC).

THC can be detected in the body for weeks after cannabis consumption, meaning users can face fines and loss of their licence, despite being unaffected by the drug.

The research, published in Neuroscience & Behavioural Reviews, analysed 80 scientific studies on the effect of THC on driving performance conducted over the past 20 years.

Danielle McCartney hopes her study will help to inform cannabis users and policymakers.(Supplied)

It found the exact level of impairment depended on the dose, whether the THC was taken orally or inhaled and how often the person used the drug, among other factors.

“Our analysis indicates that impairment may last up to 10 hours if high doses of THC are consumed orally,” the study’s lead researcher Danielle McCartney said.

“A more typical duration of impairment, however, is four hours, when lower doses of THC are consumed via smoking or vaporisation and simpler tasks are undertaken.”