A teenager girl from St Helens was taken to hospital after taking an ecstasy pill.
A 15-year-old lad was arrested in connection with the incident.
He is being investigated over claims he supplied the illegal substance.
The 14-year-old girl is now at home recovering from her ordeal.
Det Insp Jason Pye said: “This investigation is ongoing but I would like to warn people about possessing and supplying any controlled drugs, no matter how small the quantity, as you are not only putting the health of other people at serious, you risk getting a criminal record which could impact on your prospects of gaining employment in future.
“We will act on any information given and proactively target those we suspect of dealing drugs.”
Anyone with any information about the supply of illegal drugs should contact police on 101, Crimestoppers on 0800 555 111 or for further information about drug use and abuse call FRANK on 0300 123 600.