Three people needed emergency treatment after taking suspected ecstasy tablets, police said.
Officers were called at about 01:00 BST to reports of a concern for safety and found the group on Fylde Road, Preston.
Two men and a woman were taken to hospital after they were believed to have taken pink pills marked with a Mastercard logo. They have since been discharged.
Police arrested two people on suspicion of supplying a controlled drug.
A man, aged 20 from Darwen and a 19-year-old woman from Crewe, Cheshire, remain in custody for questioning.
Lancashire Police has appealed for information and has urged people not to take the pills.
Det Insp Roger Ashcroft said: “Fortunately it appears that none of the individuals involved were seriously affected by this incident and I want to reassure the public that we are trying to identify the source of the tablets.”