A 19-year-old woman and 22-year-old man are in a critical condition in hospital after suffering suspected MDMA overdoses at a festival, police said.
Officers said a number of other people who attended the Reminisce Festival at Sherdley Park, St Helens, have been taken to hospital with suspected overdoses.
Merseyside Police said the affected people are believed to have taken MDMA - also known as ecstasy - and further inquiries are being carried out.
The one-day festival which cost £85 included performances from 2 Unlimited, Cascada and Angie Brown.
On Saturday, the festival's Twitter account posted a warning about "some 'bad' blue diamond pills".
The message said: "We love to see you having an awesome time at Reminisce, but be safe!
There has been information about some "bad" blue diamond pills.
"If you are concerned about you or a friend approach some help from stewards or present to one of the onsite medical centres."
Police said the event organisers are supporting the police and urged anyone with the tablets not to take them.
Detective Inspector Paul Grounds said: "These are the early stages of our investigation but I want to reassure those affected that we are doing everything possible to identify exactly what substance or substances have resulted in these severe reactions, working alongside the health service.
"We initially believe that ecstasy tablets were taken by some of those in hospital but this is all subject to further enquiries. I would urge anyone else who has had an adverse reaction to any substance make themselves known to medical services as soon as possible.
"Likewise, if you are still in possession of any drugs from the event, please do not take them under any circumstances but let us know and we will recover them for further enquiries to be made."
Anyone with information is asked to call police on 101 or Crimestoppers anonymously on 0800 555111.