A 16-year-old boy is in intensive care after taking what may have been a Mortal Kombat ecstasy pill, police said.
Tests are also being carried out on a 17-year-old who attended the same house party after he too fell ill. The younger teenager is said to be in a serious but stable condition.
Both teenagers were taken to Monklands District General Hospital in Airdrie at about 11.30pm on Saturday after apparently taking the drugs at a house party in Coatbridge.
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Detectives and health authorities have renewed warnings over the drug. It comes after fears over the red dragon-stamped pills first emerged last month when schoolgirl Regane MacColl died. She had been suspected of taking one of the tablets at Glasgow nightspot The Arches.
Detective Chief Inspector Paul Livingstone, of Coatbridge CID, said: "At this time our priority is to ensure everyone who was at the house party is safe and well and officers are carrying out extensive inquiries to trace everyone involved.
"I would again strongly advise people to avoid illicit drugs as their exact content is unknown and can contain dangerous chemicals. Users must be aware of these dangers and understand the devastating effect they can have."
Dr Neil Howie, NHS Lanarkshire's consultant in emergency medicine, said anyone who had taken the drug and began to feel unwell should seek medical help.
He said: "We cannot stress highly enough that anyone who takes an illicit drug is putting their health, and indeed their life, in jeopardy. Users can never actually be sure of what they are taking and what the contents are."