By 1944, Howard's energy began to wane. Films like Idle Roomers (1944) and Booby Dupes (1945) present a Curly whose voice was deeper and his actions slower. It is believed that he suffered the first of many strokes between the filming of Idiots Deluxe (October 1944) and If a Body Meets a Body (March 1945). After the filming of the feature length Rockin' in the Rockies (December 1944), he finally checked himself (at Moe Howard's insistence) into Cottage Hospital in Santa Barbara, California, on January 23, 1945, and was diagnosed with extreme hypertension, a retinal hemorrhage, and obesity. His ill health forced him to rest, leading to only five shorts being released in 1945 (the normal output was six to eight per year).  The Three Stooges: An Illustrated History, From Amalgamated Morons to American Icons by Michael Fleming, states that Moe Howard pleaded with Harry Cohn to allow his younger brother some time off upon discharge to regain his strength, but Cohn would not halt the production of his profitable Stooge shorts and flatly refused his request. Other sources state the Stooges had five months off between August 1945 and January 1946. They used that time to book themselves a feature film at Monogram, and then leave on a 2-month live performance commitment in New York City working shows 7 days per week. During that NYC appearance, Howard met and married his 3rd wife Marion Buxbaum, a bad relationship that further deteriorated his health and morale. Returning to L.A. in late November 1945, Howard was a shell of his former self. With two months rest, the team's 1946 schedule at Columbia commenced in late January, but involved only 24 days work during February - early May. In spite of 8 weeks' time off in that same period, Howard's condition continued to deteriorate.  By early 1946, Howard's voice had become even more coarse than before, and it was increasingly difficult for him to remember even the simplest dialogue. He had lost a considerable amount of weight, and lines had creased his face.

Answer this question "When did he have to stop acting?" by extracting the answer from the text above.
By early 1946, Howard's voice had become even more coarse than before, and it was increasingly difficult for him to remember even the simplest dialogue.