Four people have been hospitalised after unwittingly consuming marijuana which had been added to a birthday cake as a prank.

One of the people who was hospitalised, who wanted to remain anonymous, told radio station 3AW on Monday morning the family had gathered for a birthday party and lunch when the celebration took a turn.

"Everyone was having a lovely time and cake was passed around at dessert with fruit and cream and some nice little chocolate chunks sprinkled over it," she said.

The birthday cake contained marijuana, added by a family member playing a prank.

"And it turns out the chocolate chunks contained marijuana."