I Knew Our Parents Were Lying! (page 7)
If you swallow chewing gum it will stay in your stomach for seven years
This is unquestionably one of the most pervasive myths a kid hears, and again, is based on very little evidence. While it is true that gum base cannot be digested, the idea that it will just stick to your stomach wall, essentially sitting in a vat of hydrochloric acid, is rather misguided. It simply passes through your digestive tract, alongside any other foodstuffs that cannot be digested. One note of caution, though, before anyone starts ingesting all the gum they can: There is a small risk of large quantities of gum sticking to each other, causing a blockage in the digestive tract.