Following the incident at Darmstadt Technical University, blue discolorations appeared, German police and the Hessen federal state prosecutor said in a press release on Tuesday.
A police spokesman told CNN on Wednesday that six people were hospitalized, one of whom was a 30-year-old student who was initially in danger.
Darmstadt Technical University said in a tweet on Tuesday afternoon that the six were feeling better and had now left the hospital.
One person did not need medical assistance.
Police found several containers of water and milk contaminated with a dangerous substance.
The substance is still unknown and is being examined, police said. They observed that the pungent odor of the liquids was particularly striking.
Police said the perpetrator had to have access to the kitchen between Friday 20 and Monday 22 August.
The Darmstadt University Police and Steering Committee has warned people not to eat food stored on the Lichtwiese campus where the incident took place.
Police searched university campus buildings, but so far no other suspicious objects have been found.
Police said there was no imminent threat, but urged everyone to consume only foods they controlled at all times.