Plans to allow medicinal cannabis users to drive in South Australia have been labelled "crazy" by the Police Minister.

The state's Upper House voted in favour of the amendment put forward by Dignity Party's Kelly Vincent to Labor's Drink and Drug Driving Bill yesterday.

Ms Vincent said the proposal would only apply if a doctor said the motorist's driving would not be impaired by the prescribed medical treatment.

Last year, the Federal Government legalised the prescription of medicinal cannabis and earlier this year South Australia slashed red tape allowing doctors to prescribe the treatment.

"It's no longer illicit to use medical cannabis, it is legal and we need to move this conversation forward so that we understand that a positive test does not always equal impairment," Ms Vincent said.

"We need to show trust in the medical profession that we trust their judgment when it comes to assessing people as to whether they are fit to drive or not."

Greens MLC Mark Parnell said the amendment was not a "get-out-of-jail-free card" but it would give people the ability to go to court and explain their circumstances.

Test unable to detect amount of THC in system

Police Minister Peter Malinauskas attacked the "good intentioned" amendment and said it was "inconsistent" with road safety objectives and labelled it "crazy".

"It is getting to the point where more people are dying on our roads because of drug consumption than there are of drink drinking," he said.

"So this needs a response, hence the Government has opened up the act and [are] making a number of changes."

Currently, drug testing only detects the presence of tetrahydrocannabinol (THC) in a person's system and not the amount like alcohol testing.

He said the Government supported the medical treatment but needed to keep road users safe.

"Our objective here is to make sure that people who get behind a wheel of a car have their brain operating in such a way that doesn't cause a risk to everyone else on the road," Mr Malinauskas said.

"What we know from a range of research from around the world [is] that ... any consumption of cannabis that results in THC being in your system impairs your ability to drive and therefore you shouldn't be getting behind the wheel of a car and putting every other road user at risk."

Mr Malinaukas said not all medicinal cannabis had THC but many did.

He also said some people might decide to take greater doses of the drug than the prescribed amount.

The bill still needs to be considered by the Lower House.