More than 20 people were taken to hospital after overdosing at a Melbourne music festival.

Ambulance Victoria treated 25 people at the Electric Parade Music festival, taking 21 of them to hospital. It is believed they took the synthetic drug GHB at the Myer Music Bowl.

Many of those treated were described as being in a critical condition.

"The majority of those treated by paramedics had overdosed on GHB," Ambulance Victoria State Health Commander Paul Holman said.