The funeral party went to a restaurant after the burial in Wiethagen for coffee and cake, a German tradition. After eating the cake, 13 people reported experiencing nausea and dizziness and needed medical treatment.
The Rostock police said the person in charge of the cakes asker her 18-year-old daughter to bake them.
The mother mistakingly gave the wrong cake to the mourners. She instead took the hash cake, which was made for a separate occasion.
The teen is under investigation, the police said.
Police said the incident happened in August but was not made public out of respect for the mourners.
The Associated Press contributed to this report.