BRISBANE, Australia — Lindsay Carter had his first violent seizure at 14. He toppled face first onto a tiled floor in front of his father.

At 19, Mr. Carter can still have seizures several times a month that can convulse his body and threaten his life. But more often, the recent high school graduate experiences “focal seizures,” which temporarily arrest his ability to speak or comprehend.

When he gets what he describes as “clouds in my head,” he turns to a legal but difficult to obtain remedy — medical marijuana.

The teenager has become in recent years a reluctant poster boy for the drug’s medicinal use in Australia. Appearing in news reports and radio programs broadcast across the country, he tells a story that exemplifies for many the absurdity of the nation’s marijuana regulations.