Why does this tool only work first 18months?

EXACTLY !!!

The "tool" always works. And I would ignore most of what is repeated on Facebook groups.

Part of our small intestine is bypassed with RNY. So, initially, fewer calories are absorbed, as the intestine is shorter. But our innards get more efficient at absorbing calories, and ultimately we absorb all the calories we are eating again -- with the exception of some of the fat we eat, which continues to pass through.

The common myth is that, magically, we start absorbing all of those calories again after 18 months, or a year, or whatever the myth teller passes along.

What actually happens is that our bodies start absorbing more calories again, but it starts happening gradually, from Day 1. And the number of calories not absorbed is never very large, either. It's best not to count on that malabsorption, at all. Figure you are absorbing all the calories you are eating, right from the beginning.

The reason people start gaining after 12 or 18 months has nothing to do with the "tool," it's because of what's between our ears. We are absorbing more calories because we slip back into old habits and eat more calories, not because malabsorption has come to a screeching halt.

BUT. Specific parts of the intestine absorb specific nutrients, all issues of calories aside. In particular, the location where most iron is absorbed has been bypassed. Several other vitamin absorption sites are bypassed as well. Even though our intestines adapt, and become more efficient at absorbing calories over time, the rest of your intestine cannot adapt to absorbing iron and vitamins. This is why we have to be vigilant, and take those supplements for life.

The further from surgery we get, the more we can eat and the more our body absorbs. Plus people think they are "cured" when they get to goal and they stop doing the things they did to get to goal.

I am 10 years out and 10 pounds above my personal goal weight now. I know many people who had surgery the same time as me. Many have gained back more than half the weight they lost because they went back to their old habits. A few even gained all of their weight back.

The mental part is the hardest all along the way. Remember that this is not "magic". If you do what you know you are supposed to do, you will lose the weight AND keep it off. If you choose to make excuses, you can and will gain it back. The choice is yours.