Legal cannabis could puff up the impact of chronic pain and blunt the strain on the criminal justice system, an inquiry heard, but doctors took a pot shot at the proposal, saying it could also risk exacerbating self-medication.

Parliamentarians got into the weeds of Greens senator David Shoebridge's bill on Wednesday, which would allow for cannabis possession for personal use as well as set up a national agency to register strains and regulate the growing of plants.

The young, particularly First Nations people, were over-policed as were poorer suburbs, Greg Barns SC from the Australian Lawyers Alliance said.

Most cannabis charges before the courts were for small quantities for personal use, which cost the system tens of millions of dollars a year, Mr Barns told an inquiry into the bill on Wednesday.

Just because people don't go to jail "doesn't mean there's not a punitive effect" as it still leads to legal costs and people losing their jobs or not being able to secure employment, he said.

Liberal senator Paul Scarr questioned whether there could be an increase in mental health issues, particularly in young people, from side effects including anxiety, panic attacks, psychosis or long-term memory loss.

Inadequate mental health services could be put under more strain if there was an increase in use, he contended.

But the answer wasn't criminalisation, Mr Barns said.

"People don't present to mental health services, they don't present to hospitals and one of the reasons they don't is because they are scared," he said.

It also caused a lot less harm than alcohol, he added.

Cannabis dispensary owner Malini Sietaram slammed laws for inhibiting access and the medical system for making prescriptions hard to come by and expensive to fill.

"We're almost driving people to the parking lot to get it illegally," she said.

The Australian Medical Association argued legalising cannabis across the board could lead to people self-medicating without presenting to medical professionals, leaving people "unsure of the impact on them individually".

Legalisation would lead to young people seeing cannabis "as a safe and normal thing to do, as opposed to it still being a significant psychoactive compound," Dr Michael Bonning said.

There had been increased reports of overdose and toxicity by minors, Chief Medical Advisor Professor Robyn Langham said.

Cannabis use far exceeded the risks with users overwhelmingly using it irregularly and in small amounts, drug researcher Professor Nicole Lee said.

Serious side effects in frequent users only happened to a small number of people and a well regulated market, including dosage, could further reduce risk with black market additives and highly potent strains increasing harm, she said.

Low taxes on regulated marijuana to undercut the black market and measures to reduce youth use could also reduce risks, Professor Jenny Williams said.

There had been more than 600 adverse reports of medical cannabis products since 2016, the most common being headaches, nausea, diarrhoea, dizziness and sleepiness, the Health Department's Nick Henderson said.

The inquiry will report by May 31.