I need to GAIN weight

I am struggling with my weight due to a new mediciation I need to be on. It works so well that I don’t want to switch, but DR is threatening to due to my wt loss. I have only lost 5 lbs, but have a high metabolism, so have always been just at or under what is considered the ‘healthy’ weight for my height. I have tried protein shakes etc but am hindered by my allergy to chocolate and IBS which rules out a lot of caffine or more than one fatty meal a day (ie McDonalds). The protein shakes are hard for me to get down due to taste and texture. Any other ideas out there?