text: why can't i gain wieght? i've been trying for more than 6 months to gain at least 5 pounds.  i'm only about 100lb, and i am soooo skinny.  i want to gain weight.  does anyone know how?  i have a really high metabolism.  it is hard to gain weight and keep it.  the most i've ever weighed was 103lb. i am the exact same way!  i have been 100 lbs for so long...doctors have ran thyroid tests and other tests also.  have you had your doctor test you for anything that could be the cause?  i was just diagnosed a year ago with lymphocytic colitis - my body has a hard time retaining nutrient and water.  my doctor now thinks this may be a reason why i can't gain weight.  if you have something wrong with your gastrointestinal tract, it may be a cause.  my doctors haven't really ever told me a way to gain weight - the only thing they've said is to go on a "high calorie diet" - basically just eat, eat, eat (healthy stuff of course, not junk food completely).  i've gained weight doing that...not a whole lot and it takes a long time - so have patience; it might not happen in even a year.  good luck!