Fake sick days add up. Eventually nobody believes your real ones.

Credibility works like a shared account, where every false claim is a withdrawal and the balance stays invisible until you need it. When something real happens, the response you get has been shaped by every previous instance. Nobody announces that they have started discounting you. You simply notice the sympathy is thinner than it should be.

Use the honest reason when you need a day, which is accepted more often than people assume. Ask for the time you actually need rather than building a cover story. Keep the real ones believable by not spending them in advance.