One in 20 Kiwis use cannabis for "medical purposes", but 70 per cent of those also use it for recreation, researchers say.

A study published in the New Zealand Medical Journal on Friday found five per cent of Kiwis over the age of 15 had used cannabis for medicinal purposes in the past year.

Of that group, about 70 per cent said they also used the drug recreationally.

Researchers from the University of Victoria, Wellington drew on representative data from the New Zealand Health Survey 2012-13 to find out more about medicinal cannabis users to better inform health professionals.

READ MORE:

* Cannabis legalisation is our only real option

* The economics of legalising New Zealand's marijuana market

* New Zealand urged to grow cannabis for medicinal purposes

* Medicinal cannabis likely in New Zealand by 2016

The findings supported making cannabis legally available by prescription only, giving doctors the ability to minimise potential harm, the study authors said.

Getting accurate information about how and why patients used cannabis was important for GPs, but difficult due to the drug's illegal status, they said.

A Green Party survey of GPs in 2003 found 20 per cent knew they had patients who used cannabis medicinally.

The survey also showed 32 per cent of GPs would consider prescribing medicinal cannabis products if they were legal, and 10 per cent of doctors had patients they felt could benefit from cannabis.

The New Zealand Health Survey data revealed 40 per cent of those who had used cannabis medicinally in the last year did so because of pain, while 27 per cent used it for anxiety, 26 per cent for depression, and 11 per cent for nausea.

Both medicinal and non-medicinal users reported physical or mental health harm from their use of cannabis, but a significantly larger proportion of those using the drug medicinally reported mental health harm.

The study authors said that suggested medicinal cannabis users would need monitoring for mental health problems and they supported the decriminalisation of medicinal cannabis use.

"Making medicinal use of cannabis legal, but only with a prescription, may help reduce the harm to medicinal users as they would be able to get advice about how much cannabis to use, and safer methods of use, such as the use of a vaporiser or as a tea."

It would depend on doctors having knowledge about safe cannabis use though, they said.

The study also found that cannabis users were more likely to be male.

Among those aged between 15 and 24, nine per cent said they used cannabis for medicinal reasons, and 14 per cent had used it recreationally.

Maori had the highest prevalence of medicinal use, although New Zealand Europeans, by sheer weight of numbers, were more likely to be medicinal users.