Toxicology results from tests on little blue pills found in Maidenhead have indicated they contained the prescription medication Phenazepam.
In September, a teenager was hospitalised after taking the pill with cannabis, prompting the Royal Borough and Thames Valley Police to issue a warning about its dangers.
Phenazepam is legal when prescribed, but is classified as a class C drug when it is not.
Cllr David Coppinger, cabinet member for adult services and health, said: "I am relieved that we know what the 'little blue pills' are.
"It will enable the Drug and Alcohol Action Team (DAAT) and Thames Valley Police to take appropriate action if other young people experiment with Phenazepam or are found in possession of it."
He added: "Young people need to be aware that it doesn't matter if the drug is legal, a prescribed medication or illegal, each person's reaction is different.
"Incorrect use of drugs can have serious consequences, especially if mixed with other recreational substances such as cannabis or alcohol."
Young people or parents with concerns can contact the DAAT on 01628 796518
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