I have what the allergist I was sent to as a child referred to as “atypical drug reactions”. They sent me to an allergist essentially as a last resort, since I kept almost dying when I was a small child. My mother noticed that I generally almost died after having been treated for various childhood maladies with OTC drugs. I’d get a random childhood disease, she’d treat it, and then shortly thereafter, we’d be on our way to the emergency room post haste. It took a while for her to realize it was the drugs and not just the original illness getting suddenly a lot worse - or not being the mild malady she thought it was. It took the doctors even longer - since they tended to only see me when I was already deep into the “trying to die” phase or after I’d recovered and was perfectly healthy. The allergist was essentially to settle the argument between my childhood doctor and my mother. Turns out my mother was right - and mostly because she, her siblings and her mother all have atypical drug sensitivities to the same family of drugs that were causing my troubles. Just not anywhere near as severe as mine.
Scratch tests come back negative, but if I take an NSAID (or any of several other fairly common and some uncommon drugs - I have a list, which I’ve memorized, and over the years have added to the hard way several times), I suffer an immediate and very, very severe (in some cases it’s been life-threateningly severe) reaction. It’s a totally consistent, 100% repeatable reaction - but it’s not technically an allergy. Calling it an “allergy” is shorthand. Everyone understands that allergies can be life-threatening - in my case, taking one of the drugs on my prohibited list can be life-threatening. So even though what I have isn’t technically an allergy, I think of it that way and call it an allergy when discussing it with people.
Fortunately for me, it’s at least the sort of thing where it’s childishly easy for me to avoid triggering a reaction. I haven’t had a problem in almost 15 years. Unlike food sensitivities, nobody randomly sticks my triggers in appetizers or hides them in casseroles.* Although it gets to be a pain when rattling off the list every time I see a new doctor for something. Also fortunately, there are no drugs on my list which are likely to be administered in an emergency situation (i.e., one where I or my proxy aren’t in a position tell people not to) so I can avoid the whole medic-alert bracelet fashion statement.
Weirdly, the topic of my “allergy” comes up more often than you’d think - particularly since one of the drugs that triggers my reaction is THC, and I’m sensitive enough that being near someone who’s been toking up in their car over lunch break can set off a full-blown reaction. I’m better than a damn drug sniffing dog. I haven’t been able to attend a professional concert for years. Which is a shame, since I really like music 
Also, when you have a headache or the sniffles, people want to help out and they tend to offer you OTC drugs. The vast majority of which I can’t touch. So saying “No, thank you, I’m afraid I’m allergic to aspirin/Advil/Tylenol/etc.” is just easier than trying to explain the “atypical reactions” dealie.
*My profoundest sympathies go out to anyone who’s ever had to say “Oh shit! Were there nuts/shellfish/etc in that?” as the hiving starts up. My best friend has terrible, terrible food sensitivities (and actual allergies as demonstrated by repeated scratch tests - she visits an allergist every year since she keeps adding more things to her allergy roster) and I’ve lost track of the number of times I’ve seen her lunging for the Benadryl because of an incomplete food description on the menu.
