Air Carrier Captain reported a fume event during climb. After initially dissipating the odor resumed resulting in multiple complaints. Flight crew executed a safe air return to the departure airport.

Narrative

Air return due to odor reported by Flight Attendant; multiple customers; deadheading crew; and flight deck. Cabin reported the odor when I contacted them to ask them to stay seated on climb out. The First Officer also inquired regarding an odor. I also smelled something but it was faint at that time. Odor increased on climb out then dissipated but later increased to the point customers were complaining. Aft Fight Attendant stated the odor was increasing and prompted a second flight deck call. I contacted Dispatch and began reviewing the Flight Crew Operations Manual. Odor intensified with water ingestion. Numerous customer complaints as well as Flight Attendants in both aft and forward cabin stated it was increasing while we were reviewing options with ispatch. One customer requested medical attention. Odor was present throughout entire cabin and cockpit and we were unable to isolate the source. We decided to return to ZZZ at that point. Completed all checklist and landed safely in ZZZ.