About 250 people have gathered in Perth and around Australia calling for governments to legalise cannabis for sick and dying patients.

Five-year-old Madison Graham has up to eight severe seizures every day and must wear a helmet to protect her head from injury when she falls.

Madison's family hopes that if cannabis is legalised, it can be used to help treat her illness.

Her aunty, Lisa Gliddon, said doctors don't know what causes her seizures but think it's likely to be a rare form of epilepsy.

"She will be standing and then suddenly drop to the ground," Ms Gliddon said.

Madison carries an oxygen tank with her because her seizures are so severe she stops breathing.

"The family is desperate to have some normality in her life," Ms Gliddon told AAP.

Madison and her family attended a rally of about 250 people at the WA Supreme Court Gardens on Sunday, urging the state government to make marijuana available to those who are sick and dying.

Angel Watterson, 13, has juvenile psoriatic arthritis and epilepsy, and told AAP she was hopeful that cannabis would help treat her illness and lessen her pain.

"I just want the government to listen," she said.

WA Health Minister Kim Hames said in a statement that different state and territory positions on therapeutic goods laws, as well as obligations to international conventions, made it a complex issue.

"There is evidence supporting the benefits of cannabis and its extracts in the treatment of a range of conditions. But the evidence still needs to be of sufficient scientific quality to definitively answer questions about efficacy and safety," he said.

Dr Hames said he knew of pharmaceutical versions of cannabis derivatives, including an oral spray that could offer medical benefits without the risks of smoking cannabis.

"Research into these potential medicines should be strongly encouraged but we need a national approach," he said.

Other rallies to promote legalisation of cannabis for medicinal use were also held in other capital cities.

Copyright 2014 AAP.