The Big Bang Theory originally gave Sheldon Cooper a cat allergy, but future seasons seemingly cured him through a retcon. Jim Parsons played the brilliant scientist Sheldon Cooper for all 12 seasons of The Big Bang Theory. The story revolves around Sheldon moving into an apartment with his best friend Leonard Hofstadter (Johnny Galecki) and their usually entertaining day-to-day lives. Over the course of its long-running story, The Big Bang Theory advanced the stories of Sheldon, Leonard, Howard (Simon Helberg), Raj (Kunal Nayyar), Penny (Kaley Cuoco), Amy (Mayim Bialik), and Bernadette (Melissa Raunch) in major ways.
Sheldon Cooper was always one of The Big Bang Theory‘s most particular characters. Despite his genius-level IQ, Sheldon is socially awkward and displays idiosyncratic behavior. The various quirks of Sheldon manifest themselves in different forms throughout The Big Bang Theory. He must always sit in his spot on the couch and makes Leonard sign a Roommate Agreement that outlines specific and unique rules of their living situation. With a character as detail-oriented as Sheldon, many fans of The Big Bang Theory were surprised to see the series eventually retcon one of his supposed allergies.
The Big Bang Theory established very early on that Sheldon is allergic to cats. The third episode of season one sees Leonard adopt a cat because he is feeling lonely after being rejected by Penny and Leslie Winkle. Sheldon opposes this idea and it is highlighted that the presence of the cats could trigger Sheldon’s asthma. Although the cat does not stick around for the duration of The Big Bang Theory, Sheldon’s allergy to cats is still established. However, The Big Bang Theory retconned away this allergy only a few seasons later and appeared to cure Sheldon.
The cat allergy retcon comes in The Big Bang Theory season 4 episode 3. The episode shows Sheldon and Amy temporarily break up, which leaves Sheldon in need of comfort. Sheldon takes this opportunity to bring 25 cats into his and Leonard’s apartment as a way to make him feel better. He is noticeably happy to be with his new pets but it is also clear that he is not allergic to them. If one cat was enough to worry Sheldon before, he never would’ve put himself in the presence of over two dozen. It appears The Big Bang Theory cured Sheldon of his cat allergy all for the sake of this short-lived storyline.
That said, there is another possible explanation for Sheldon’s cat allergy disappearing: he never was allergic to cats. Sheldon is notoriously selfish and will do whatever he can to get his way, especially early on in The Big Bang Theory. It is possible that he tried to use allergies as an excuse to keep the apartment the way he wanted at the time. This wouldn’t be too surprising of an explanation and clears up one of many Sheldon-related retcons The Big Bang Theory is responsible for. Since the show is over, it might be up to Young Sheldon to clarify if Sheldon was ever allergic to cats to begin with.