A Sydney man is battling a rare form of meningitis after reportedly eating a slug as a dare.

Health authorities today warned of the dangers of eating the creatures after the man contracted rat lungworm disease, the ABC reports.

The disease is caused by a parasitic worm carried by slugs and snails.

The man is reportedly in a critical condition in hospital.

Dr Jeremy McAnulty from New South Wales Health told the ABC that cases of rat lungworm disease were rare and most people could recover.

"It goes away by itself because the body's immune system will eventually get rid of it," he said.