Image copyright GOOGLE Image caption The girl collapsed on Meneth Road in Camborne, police said
Sid Willett, from the Cornwall Drug and Alcohol Action Team , said the substance involved could cause "fitting, overheating and severe irritation".
He told the BBC: "You can bet your bottom dollar this drug could be in every corner of the UK.
"It is possible these tablets could have come from abroad or on the dark web".
Among tributes paid to Miss Pellow on Facebook, Kathryn Yeoman said: "I am shocked!... Such a lovely young lady gone far too soon."
Jessie James said: "She had an amazing soul and was so beautiful... She will be missed by so many people."
Victoria Soares described it as the "saddest heartbreaking news ever".
Two 17-year-old boys, one from Camborne and the other from Redruth, were arrested on suspicion of being concerned in the supply of a controlled drug. They were later released on bail, officers said.