Isn't a little bit of poison good for you ?

I keep hearing that people who have been in ketosis for years, pooped out their pancreases after it atrophied and fell off. 

Joke aside, are there any long term dieters who felt ill or the like, just because they had a cookie ?  Anyone cycling just to avoid this ?

If keto means I get allergic to carbs in the long run, dizzy if I drink a coke once in a year, I will definitely won't maintain ketosis indefinitely.