A bio-hazard alert was issued at a South Yorkshire metal detecting convention after treasure hunters were given chocolate cake spiked with cannabis.
At least 15 metal detecting enthusiasts were taken ill at the event in the village of High Melton, near Doncaster, on Saturday evening, and 13 of those had to be taken to hospital.
NHS bosses deployed a Hazardous Area Response Team who are trained to respond to serious public health threats, including terrorism.
A South Yorkshire Police spokeswoman said: "An investigation is underway after a group of people fell ill as a result of eating something reportedly containing cannabis, during an event in the High Melton area of Doncaster.
"The group, of both men and women, were taken to hospital at about 7pm on Saturday 24 August. All have since been discharged.
"A 48-year-old woman from County Durham has been arrested in connection to the incident and currently remains in police custody where she will be spoken to by detectives."
A Yorkshire Ambulance Service spokesman confirmed 15 patients were treated by ambulance at the scene and 13 of those were taken to hospital for further care.
He said: "We received an emergency call just before 6.30pm on Saturday (24) to reports of a number of people feeling unwell at a national metal detecting event being held near Sprotbrough, Doncaster in South Yorkshire.
"They had become unwell after ingesting an unknown substance.
"Eight ambulances, a rapid response vehicle, two doctors, two clinical supervisors and the Trust's Hazardous Area Response Team were dispatched to the scene.
"Fifteen patients were treated by ambulance service staff on scene and 13 of those were conveyed to hospitals in South Yorkshire for further care."