Andrew Starr wrote: What is the etiquette of informing
someone you are lactose intolerant?

When recently invited to a dinner party I said:
"I can't eat milk, will you be serving anything with milk or
cheese in it?"
The hostess was making a pasta dish with cheese, so she made
me some plain pasta and I brought a jar of commercial sauce.
When she started making the salad, I took some greens out
before she added parmesan and creamy salad dressing.
Unfortunately the Lemon Ice-Cream Baked Alaska Pie was out
of the question. Sigh.