Why is my cat shedding so much in winter?

Because indoor temperature and lighting don’t change much through the year, the seasonal shedding cycle that drives outdoor cats flattens out for indoor cats. Indoor cats shed steadily across all twelve months, and winter shedding can feel worse than it is because of dry air making the loose hair more static and visible on dark clothes. Brush more often (daily for long-haired, twice a week for short-haired) and vacuum more than feels reasonable. There is no supplement that materially reduces shedding in a healthy cat. The body sheds the hair it sheds.

When to call a vet: if the heavy shedding comes with bald patches, scabs, weight loss or behavior changes, that’s a vet visit, not a brushing problem.