Victoria's two newly elected upper house MPs from the Legalise Cannabis Party introduced a bill on Wednesday to change the Road Safety Act.

Under the legislation, it would no longer be an offence for unimpaired drivers to have detectable tetrahydrocannabinol (TCH) - the primary psychoactive substance from cannabis - in their blood or oral fluid.

They would have to carry a medicinal cannabis prescription and be taking the drug as prescribed to be legally permitted to get behind the wheel.

In 2016, Victoria became the first state to approve medicinal cannabis but unimpaired users were not exempted if caught driving with the drug in their system.

"People who have been prescribed a medicine and can drive safely should be allowed to drive," Legalise Cannabis MP David Ettershank said.

"This is how we treat every single prescription medicine in Victoria, except medicinal cannabis, and its time for that to be corrected."

Medicinal cannabis patients can drive in Tasmania if unimpaired and fellow Legalise Cannabis MP Rachel Payne called on Victoria to follow suit.

THC is known to stay in saliva for between a few hours and several days after use and is commonly found in the blood and urine samples of frequent users weeks after the initial effects have worn off.

Alice Davy, who has been diagnosed with multiple sclerosis and endometriosis, said medicinal cannabis saved her life.

"Without this I wouldn't be able to get up in the morning to take my kids to school. I have THC at night and CBD (a nonpsychoactive component of cannabis) all day," she said.

She used to take prescription benzodiazepine and would wake up so impaired she was unable to drive.

"That's a completely legal drug and cannabis isn't," Ms Davy said.

Karen Hitchcock, a medicinal cannabis doctor, said many of her patients opted not to take medicinal cannabis to avoid the risk of losing their licence for six months.

"They can't drive their children around; they can't get to work," she said.

"I'm giving them care based on what are draconian rules that aren't actually evidence based or science based."

Premier Daniel Andrews said it was a challenging area of policy as there was no test to gauge a cannabis user's level of impairment, but the government was open to exploring solutions.

"I know its deeply frustrating for those who have advocated for some time," he said.

"The aim is to make sure that everyone for whom medicinal cannabis is an important part of their treatment regime ... (to) have no barriers to accessing that care."

Australian Associated Press