Reaching for a glass of OJ may be your first instinct when you feel sick—all that vitamin C, right? But citrus fruits, including oranges, grapefruit, and lemon, can irritate the stomach lining and aggravate an upset stomach. If you’re feeling nauseous, best to stay away until your stomach settles down. Dr. Sulapas suggests taking an over-the-counter vitamin C supplement rather than risking stomach upset.