What is the best way to gain weight?I'm 5'5in and I weigh 120 lbs. and I would like to weigh 140 but I'm having a hard time adding on the pounds. I would like to know if there is a certain vitamin that I could be taking or what certain foods should I consume. Any replys are welcome. Thank you:)

First and foremost, you have to gain weight healthily. This means... NOT binging on fries + chips + sodas...

Overall, at your frame, you're burning about 2200 calories a day (obviously, I don't know your lifestyle... your daily routine... exercise routine.. etc. so I'm estimating).

In order to gain weight, you just have to eat more than 2200 calories a day. This includes eating regularly (every 3 - 4 hours) and just eating "healthy fatty foods" such as:

Legumes
Nuts
Peanut butter
Meat
Turkey
Chicken
Pasta

etc. Be sure to keep a regular eating schedule... don't overstuff yourself. Also, try to maintain a moderate exercise routine... as muscle weighs more than fat.

Best wishes.