For instance, clover honey gives me a headache, but honeys made from other sources generally don't.
For a long time now I've had a problem with apples giving me a headache. Sometimes I could eat them with no problem, and I could tell by smelling them whether or not I'd have a reaction. Strange, I know
Recently my parents tried a new apple called Envy, which I had no reaction to whatsoever! I've been having an apple a day for the past 8 days or so, but today was different. Same batch of apples, but today's was different -- it gave me a headache.
There was only one difference between this apple and the other apples, and that is that the apple had been sitting around for a while.
An answer to the weirdness of sometimes-yes, sometimes-no at last! There has to be some kind of substance that builds up as they ripen, and that's what's been causing the headaches.
This was probably boring for those without food sensitivities, but it's a big deal for me. Now I know that if it's not overripe I can eat apples whenever I like, and I'm super-happy about that