Police say an unknown brown powder being sold as the drug MDMA has been linked to three hospitalisations in Whanganui over the weekend.
The three "serious harm events" occurred within 12 hours on June 5, and are warning the public of the brown powder being sold in small plastic snap-lock bags, police said in a statement.
The symptoms identified were not consistent with a typical MDMA overdose, leading police to believe the substance had been adulterated or misrepresented.
High Alert, a website that reports the circulation of dangerous drugs in New Zealand, strongly urged people not to take any unknown powders or tablets and recommended getting them tested to help lower the risks.
MDMA, or ecstasy, is a class B drug in New Zealand.
It is unknown which substance was responsible for the harm, but reported symptoms had been rapid heart rate, vomiting, difficulty breathing, faintness, heart palpitations and loss of consciousness.
Whanganui District Health Board chief executive Russell Simpson said these events were concerning.
"It has an impact on not just individuals but the families and others around them," Simpson said.
"It also consumes valuable resources which could be utilised for other things."
Executive Director of the NZ Drug Foundation, Sarah Helm, said drug-checking offered some protection from an otherwise unregulated illegal market, but the limited availability of equipment meant the service wasn't readily available in places like Whanganui.
"Home 'reagent' tests look for the presence of a substance, but don't tell you what else it may be mixed with," Helm said.
"Generally one of the main risks of MDMA is what it can be substituted for or adulterated with."
Helm said if people were planning to consume MDMA it was safest to take a small dose as it could take up to an hour to feel the effects.
Any unexpected or concerning effects from drugs can be reported through High Alert.
Anyone with information in relation to the manufacture and supply of illicit substances is asked to contact Whanganui Police on 105 or anonymously via Crime Stoppers on 0800 555 111.