Medicinal cannabis oil may not be enough to save the life of a Nelson teenager.

Associate health minister Peter Dunne has given one-off approval for the 19-year-old to receive cannabis oil.

Wellington Hospital staff have already tried more than 40 medications to treat Alex Renton since he was put into an induced coma two months ago.

Mr Dunne doesn't expect the oil to work, and Drug Foundation executive director Ross Bell says he's probably right.

"There is guarantee this product, because it's new, will work on this case, because it's rare."

Ross Bell says there are many other patients who may benefit from medicinal cannabis, and this case may force other cases to be reconsidered.

Peter Dunne [told Larry Williams] says he made the decision based on advice from doctors, and the case shouldn't be construed as setting a wider precedent.

But Ross Bell says the case shows there is a need for the Government to change to the law to allow limited medicinal use.

"We're grown up enough to create a regime which is compassionate, and which keeps up to date with the modern science."

Ross Bell says most people realise evidence-based medicinal use is very different from recreational use.

He says this decision won't have any bearing on the quite separate issue of whether to legalise recreational cannabis.