Thank you for your query, Would you please clarify a few things for me? 1) How much weight did she gain and in how much time? Is she overweight/obese? 2) Does she have any thyroid disorder or a family history of it? 3) Are her menstrual cycles regular? 4) Is she an active person (sports, going outdoors regularly) I understand the concerns of weight gain , especially in an 18 yr old. Weight loss is a Multifactorial concept. To make it simple burn more calories than you eat . Hypothyroidism and irregular menstrual cycles ( as in Polycystic ovarian disease) can cause weight case. These can be excluded with a blood test that checks the thyroid profile and female hormonal profile and an Ultrasound of the abdomen to look for cysts She would need to be seen by her physician. If someone in her situation were to consult me, I would like to rule out the above medical causes for the weight gain before getting started on the following. How much calories a person needs depends on their height, weight and basal metabolic rate. I would recommend a regular exercise routine (starting with cardio workout that burns calories and adding on weight training to build lean muscle which will aid in burning calories more effectively and consistently), in addition to a well balanced diet. Foods to avoid are carbonated drinks, processed and fried foods (such as pizzas, french fries, burgers), refined grains and sugars Include fresh fruits, vegetables, sprouts, fish, dried fruits/nuts, whole grains. drink plenty of water. The best way is to drink before you are thirsty and eat before you get too hungry. have 3 small meals with 2-3 snack time during the day instead of just 3 large meals. This will help to keep the metabolism going, help in losing weight and stay energetic through the day. hope this helps and please get back to us if you any further queries. Wish you good health