Gaining

I haven't gained a ton (not sure I want to yet, still feeling out if I enjoy it), but the increased calories has caused issues with my IBS. Mostly just more often getting cramps and stuff. Pretty mild as far as medical issues go, but still annoying.

my pre-existing condition isn't necessarily made worse by me gaining weight, but it does contribute. i have hypothyroidism after having a full thyroidectomy years ago. it's one of the reasons i decided to just enjoy gaining after years of fighting to even maintain my weight. life is too short to deny myself the things i enjoy like delicious food and that blissful feeling of an overstuffed belly.

I have hypothyroid decease before I was treated for it, I actually didn't really gain any weight, it caused a very low heart rate. Since I started to gain, the only change was increases my dosage, due to my increased weight. I also have acid reflux, but that is treated with prescription antacid.

Not really pre-existing conditions, but still gaining despite developing medical conditions. Honestly the hardest to work around is a gluten intolerance. Though sometimes you've just got to spite your stomach for good food.
More seriously I've got low level diabetes, high blood pressure, and high cholesterol. If I watch my intakes of sugar and salt I'm generally fairly good. My doctor is aware I'm not planning on dieting so he's got me on meds to handle most the heavy lifting there.

I have a type of eczema that flares up due to certain foods. Any simple carbs, sugary foods, high lactose, high vegetable oil, most forms of beef, high sodium or very starchy foods will make my face break out into red itchy splotches.

So I have to eat fairly healthy to work around this condition. I've still managed to find ways to eat 3-4K calories a day though so it's not that big of a hindrance.