Seven people were treated in hospital over the weekend after taking a potentially deadly drug known as "Rockstar", according to police.
The red or yellow coloured tablets, being sold as ecstasy, are known to be circulating in the Lanarkshire area.
Strathclyde Police issued a public warning about the dangers of the substance, which officers said was responsible for making a number of young people seriously ill.
All those taken to hospital after suffering adverse reactions to the pills have since been allowed home.
Chief Inspector Fraser Lamb said: "These substances are unreliable, unpredictable and potentially very dangerous. Users may believe that they have taken ecstasy, and it is very likely that they will suffer from a significant negative reaction. Symptoms reported recently include increased heart rate, elevated core temperature and seizures.
"These pills are not covered by any form of quality control and this is supported by past forensic analysis. Users need to be aware of the dangers and understand the potentially devastating effect these pills can have on their health."
The exact make-up of the pills - which normally have a star impression on them - is unknown, but police believe they may contain a cocktail of different substances.
Strathclyde Police said they are working with health officials and medical staff in connection with their investigation.
Chief Insp Lamb added: "We are continuing to take this matter extremely seriously and extensive police enquiries are ongoing to establish the source of these drugs and every effort is being made to track down and arrest those responsible for selling these drugs as quickly as possible. Additional patrols have been deployed in the Lanarkshire area."
Detective Sergeant Michael Miller said: "Our advice is consistent. Drugs bought from the illicit market can be dangerous. I would strongly advise people to avoid such pills and report any information to the police."