Three people have been arrested after several teenagers needed medical treatment for using the drug Xanax.
The teenagers are believed to have obtained and taken the "dangerous" tablets in the Upton area of Dorset, police said.
A 20-year-old woman, a 19-year-old man and a 16-year-old boy - all from the Poole area - were arrested on suspicion of supplying illegal drugs.
They have been released under investigation, police said.
The drug, also known by the name Alprazolam, can be obtained on private prescription in the UK to treat anxiety but is not available on the NHS.
Dorset Police said it could "slow the heart rate down to dangerous levels".
At least three teenagers have been medically treated for Xanax use in recent weeks, the force added.
Det Con Ben Griffin urged parents in the Poole and Purbeck areas to report any concerns.
"We would urge parents to be on the lookout for any changes in their children's behaviour or any tablets they may be found in possession of," he said.
Xanax has a similar sedative effect to Valium but is about 10 times stronger, Public Health England has previously said.
It said there was evidence to suggest that use is a growing problem, particularly among young people and young adults.
In February, BBC South East found Xanax was being sold illegally to children on social media sites.
In November 2017, the rapper Lil Peep was found dead after an overdose of Xanax mixed with Fentanyl - a powerful synthetic painkiller.