In IF, “forbidden” mainly refers to calories during the fasting window: sugary drinks, juice, milk/cream coffees, caloric flavored beverages, and snacks break the fast. In the eating window, people claim “anything goes,” but outcomes strongly depend on food quality—ultra-processed foods, excess added sugar/alcohol, and low fiber/protein undermine weight and metabolic benefits. A second category is medical safety: unplanned IF is not advised for pregnancy/breastfeeding, eating-disorder history, children/teens, or people at hypoglycemia risk (e.g., diabetes on insulin/sulfonylureas). If dizziness, fainting, palpitations, severe fatigue, or symptom flares occur, widen the window and seek evaluation.