It's an age old tale:
Girl gets hooked on drugs and checks into rehab. Girl gets clean and gains weight. Girl loses weight and just can't figure out how to gain it back. Girl checks back into rehab and rumors spread like wildfire about girl having a gastric bypass surgery reversal.
OK, so maybe it's not age old but it's definitely a tale worth noting.
For all those out there having issues gaining weight, it would be easy for everyone else to say, "Oh, go eat a donut already!" But sometimes it is just not that easy, so for some healthy tips on how to put weight on safely, read more
A good rule of thumb is adding 3500 calories to your diet over the course of a week is equivalent to gaining one pound. (Gaining more than one pound a week can be detrimental to your health.)
Say you are like Nicole and about 5'2" and 25 years old with a small build, an ideal weight should be at or around 108-121 lbs. This means that ideally you should be eating somewhere around 1600-2000 calories per day, depending on your activity level. One way to put some pounds on would be to add about 500 calories per day to your daily caloric intake (bringing you to the additional 3500 a week). The key to safe weight gain is doing it in a healthy way. For example, you could eat several Krispie Creams and get to your 500 calorie goal rather quickly, but then you may be looking at some health risks down the line (high cholesterol, etc).
Fit's tip: Raw nuts and fruit juices are a great way to get more calories in a healthy way.
And, like Nicole, always consult your doctor before beginning a new diet.