The World Health Organisation (WHO) says pesticide poisoning is the probable cause of 18 mysterious deaths in Ondo State.
Those who died in the South Western state suffered from blurred vision and headaches, and then lost consciousness before dying within 24 hours.
A spokesman for the WHO said tests done so far had been negative for viral and bacterial infections.
He said the current theory was that the deaths were caused by weedkiller.
Speaking at a press conference, the Commissioner for Health in the state, Dayo Adeyanju, says the strange disease is not contagious and could be a result of ethanol poisoning.
According to the commissioner, preliminary report showed ethanol poison found in the systems of all the victims.
“Our investigations revealed that the victims, who are commercial motorcyclists, gathered at some local joints to take alcoholic substance mixed with roots and some other local herbs on the eve of the outbreak of the disease.
“I can assure you that the disease is in no way contagious. The fact that none of the caregivers has contracted the disease has greatly underscored this point.
“Therefore, the fear of spread does not arise and should be discouraged.
“We strongly suspect ethanol poison and in view of this, we have ordered for another toxicology test for surviving victims,” Mr. Adeyanju said.
The outbreak started in the town of Ode-Irele.
All of those affected started showing symptoms between 13 and 15 April.