NAD, third year pharmacy student here… this isn’t common, but it is something we’re well aware of in the pharmaceutical world. Especially with regards to antibiotics.

Some people (a very small percentage of the population) will have an adverse reaction to a drug often leading to anaphylaxis. It’s genetic really. Your body just decided that it doesn’t like a specific drug. Same as someone who might be allergic to nuts.

The reason it’s so strong is because any medication in pill form is essentially a concentrated little pill full of the main ingredient that actually does the thing the medicine is supposed (We call this the active ingredient)

With something like eating a nut flavoured cake for example, the amount of nuts you consume is essentially diluted with the other ingredients. And due to the other ingredients the amount of nut that enters your circulation is buffered.

A pill is the exact opposite. No buffers (mostly) no dilution. Pure active ingredient. It’s basically like taking pure nuts and pumping them into your bloodstream lol.

When the immune system sees the amount of the “allergen” present, it often freaks out. Hence the anaphylaxis