What can i be prescribed to make me gain weight because i been 100 pounds half of my life and im now 26 years old. i've been drinking ensure and boost?

A matter of balance.: To gain a single pound, you must eat 3500 calories more than you use. This would translate to an increase in calories of 500/day to gain one pound in a week! continue to be sure that your extra calories are nutritious, not junk. Greek yogurt, cottage cheese, almond butter on whole grain toast, smoothies are other good examples. You probably don't get much sympathy, but it can be hard to gain!

Body composition: I recommend getting a body composition analysis to see what percentage of your weight is fat, what percentage muscle & what your hydration level is. If you're a female, body fat is 18-26% for your age(male 14-18%).If your body fat is normal don't worry about your weight, with age, it'll go up. Eat a healthy diet&don't restrict yourself. If body fat is low, then need a work-up and see a specialist.