Drug-testing service KnowYourStuffNZ discovered an increase of synthetic cathinone known as eutylone, being sold as MDMA, or ecstasy, this week.
About 40 per cent of the samples brought by people thinking they had MDMA during O-week tests have turned out to contain the substitute.
Side effects include not being able to sleep for days, anxiety, paranoia and in some cases it can be fatal.
In 2018, 13 people - including a 15-year-old - arrived at Christchurch emergency department over one weekend suffering from a bad batch of what was thought to be ecstasy.