[header] How to gain weight and muscle [title] Meet with a doctor. [step] Before you begin a new workout regimen or try a drastically different diet, you need to meet with a doctor to discuss your health. [substeps] Talk to her about why you want to gain weight and muscle mass, and tell her about the kinds of things you're going to change in your lifestyle. 
Ask your physician what a safe amount of weight gain is for your age and gender. Gaining too much weight could put your body weight into an overweight category.