Northern Territory Police say people in Darwin have experienced convulsions and vomiting after injecting a bad batch of an illegal drug.

Superintendent Tony Fuller says a number of people have reported severe reactions to the white crystalline powder, known as "crank" or "bath salts".

He says police have seized some of the batch and are running tests on it.

"The problem is that there are so many different synthetic drugs out there," he said.

"We are continually trying to keep in top of it and work out what we've got in town but this one seems to be a bit more nasty than normal.

"The intelligence we are getting is that the reactions are pretty extreme and pretty rapid."