The Northern Ireland Ambulance Service declared a major incident after up to 40 young people attending a concert in Belfast became ill.
Nine people were treated in hospital although none are thought to be in serious condition.
It is believed the majority of those affected were suffering the effects of drink and drugs.
Around 7,000 people, many of them teenagers had packed into the Odyssey Arena in the Titanic Quarter area of Belfast for a concert headlined by the Dutch DJ Hardwell.
The problems began when about 20 people presented themselves at the SOS station at the event.
Due to the numbers seeking medical attention the Ambulance Service set up a mobile treatment centre outside the arena and around 40 people were seen by medics there.