Dunedin has become a centre of people unwittingly taking the notorious psychosis-inducing drugs known as "bath salts'', because dealers are misrepresenting their wares to make a quick buck, a testing organisation says.

The comments come after St John ambulances were called to North Dunedin 13 times during and immediately after an Agnew St student party, all related to alcohol and/or drug use.

St John yesterday sounded the alarm about what it says is the growing number of people showing severe adverse effects after taking synthetic drugs.

Know Your Stuff managing director Wendy Allison, whose organisation runs drug testing at events as a means of harm reduction, said her organisation had earlier detected drugs containing cathinones in Dunedin.

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Also known as bath salts, the drugs are sometimes sold as MDMA (ecstasy) because they are cheaper and in some cases easier to obtain than MDMA, but they could have deeply unpleasant and dangerous effects, including psychosis, she said.

She had heard reports Dunedin was becoming an epicentre of drug dealers selling bath salts to unwitting customers.

"Dunedin seems to be a place where there is a lot of cathinone substitution going on at the moment.''

Cathinones, including n-ethylpentylone and eutylone, had both been found in the organisation's testing in New Zealand.

The former was particularly insidious, she said.

"The problem with n-ethylpentylone is it's similar to MDMA to start off with, but after half an hour to an hour the fun effects go away and the user is left anxious, agitated, with a racing heart, sweating, unpleasant stuff.

"People can be awake for up to three days, and if you have enough of it and don't sleep for long enough, you're going to have a psychotic episode.''

The effects of eutylone were not quite so extreme, but the subtle effect produced by the stimulant was its own issue as people took it thinking it was MDMA, felt relatively little, then would take more and become overstimulated, she said.

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She urged people to get their drugs tested before consuming them (or test them themselves, as reagent testing kits were available legally online), but not to take anything they were unsure about.

Dunedin Hospital emergency department clinical lead John Chambers said 31 people presented there during or after the party on Saturday, about three times as many as staff had been anticipating that day.

Three or four of those had suffered cuts from corrugated iron but the remainder were suffering the ill effects of intoxication, and at least 10 of those reported taking ecstasy, he said.

But he could not be sure if any of those had in fact taken bath salts.

One man had been spitting at and abusing staff, Chambers said.

St John Otago and Southland district operations manager Pauline Buchanan said St John was seeing a rise in the number of people showing "severe adverse effects'' after taking recreational drugs.

"And in particular, we are very concerned about the effects of the synthetic drugs.

"A big concern for us is that the ingredients used in each batch are usually unknown to the user and we urge people to avoid using recreational drugs or psychoactive substances in the interests of their own health and wellbeing.''

Ambulances transported eight patients to Dunedin Hospital for further assessment and treatment for conditions ranging from moderate to minor.