Dexamethasone (a steroid) and cyproheptadine (an antihistamine) can be used together in some conditions, but they should only be taken under medical advice. Dexamethasone can cause side effects like increased appetite, weight gain, mood changes, acidity, high blood sugar, and reduced immunity if used for long periods, while cyproheptadine commonly causes drowsiness, increased appetite, weight gain, and dry mouth. When taken together, they are generally safe for short-term use, but may increase sleepiness and appetite.. It is important not to use these medicines regularly without a doctor's guidance, and to seek medical advice if you notice unusual symptoms or if the medicines are needed for a long duration.