A group of 27 students were taken to the Aghios Andreas hospital in Patras on Wednesday night after developing gastrointestinal symptoms just before boarding a ferry for Italy.
The students were given first aid treatment, and 23 of them were released, while the other four were admitted to hospital.
Hospital deputy director Dimitris Lygouras told ANA-MPA that samples have been sent to the National Center for Disease Control and Prevention (KEELPNO) to determine what caused the group outbreak of the symptoms.
The students are members of a 44-person group touring Western Greece. The rest of the group is being hosted at a Patras hotel.