I can't eat lettuce (out at a restaurant - I'll chance it at home) because it has the possibility of doing mean things to my stomach.
I don't go into details (they wouldn't be pretty anyway), so I just say, "Could you make sure there isn't any lettuce on the plate - even as a garnish? I can't eat lettuce. Thanks!" I've never had anyone question it or act like it was odd.
So, that's the advice I'd give as to what to say. I've found that it's quick and simple. And it gets me what I need.