Jersey Customs officers are concerned people are importing the class B drug mephedrone under the impression it is plant food.
Officers have seized 12 packets containing the illegal drug in the past three weeks.
They said the packets had been imported "innocently" - ordered on the internet as plant food called Jolly Green Granules, so no-one has been charged.
Officers think people might be buying it without knowing it is illegal.
Customs said it had been a shock to some of those involved when officers turned up at their house to investigate.
Neil Hussey, assistant director of investigation at Customs, said that websites were selling mephedrone as plant food and in some cases describing the product as "not fit for human consumption".
Mr Hussey said: "What's clear from the websites is that there is no indication they are actually mephedrone, in fact they are trying to sell these things as plant food.
"Historically the mephedrone we have seized has been this white/off-yellow powder, but this particular batch we are seeing are pale green granules."
Michael Gafoor, from the Alcohol and Drug Service, said some islanders may have taken the drug unwittingly.
Mr Gafoor said that, although it may appear harmless, mephedrone could be very dangerous.
He said: "Mephedrone was reclassified as a Class B drug in April of this year and obviously some websites and drug dealers have been left with stocks of mephedrone to sell.
"It is being sold as a legal high, as a non-toxic and benign drug but all the evidence is that it actually contains mephedrone, which is a stimulant drug which can cause very serious side-effects, some of which may require very urgent medical attention."
Mr Hussey said that it was easy for them to spot when people had purchased the products, but there had been an increase in the number of seizures.
He said: "They appear to be coming from single sources or maybe a couple of websites and they are identical in nature so it is easy to pick them out.
"We are concerned there has been a peak in the number of mephedrone seizures over the last three weeks and we are trying to raise awareness of this to the public."