There are many medical reasons for being underweight. Before trying to put on weight, you need to know that it is safe to do so. The best way is to make sure you are supervised by someone appropriately qualified.
Check if you're a healthy weight for your height using the BMI healthy weight calculator.
If you're underweight, your GP can check for any health conditions that may have caused you to lose weight, such as:
Once the cause of your weight loss has been found, your GP can give you advice about making changes to your diet, sometimes with the help of a dietician.
The type of food you will need to eat to put on weight will depend on what is causing you to be underweight.
Read more answers to food and diet questions.
Further information:
Your health, your choices