Police in Preston have warned people of the high strength of the new "Punisher" drug.
Officers are concerned that a batch of the high-purity form of ecstasy has been smuggled into the county.
In a press release, Lancashire Constabulary warned drug users in Preston that so-called "Punisher" pills may be circulating in the city.
The pills are said to be three times the strength of ordinary ecstasy, also known as MDMA, and extremely dangerous.
Det Sgt Lee Richardson, of Preston Police, said: “We are looking into this following reports three people had suffered adverse effects after taking what they believe to be Punisher.
“Our advice would be for users to seek immediate medical attention if they notice any different symptoms or start to feel unwell.
“There is no safe way of taking drugs, and no way of knowing exactly what you are putting into your body and whether it is safe.”
Typically a single ecstasy pill might contain between 10mg and 150mg of MDMA, but "Punisher" pills contain much higher amounts.
Test data published on ecstasydata.org suggested "Punisher" pills contained around 234mg - a potentially fatal dose.
But similar pills seized by police in 2018 had been found to contain more than 300mg of MDMA.