One of my F/Officers had to observe a special diet and take extreme care due to a reaction to peanuts. His description of the symptoms was horrific.
At that time we would have been on the cusp of CAA-JAA medicals, so new applicants would need to check with their CAA for the latest rules.
Mac is correct in his assessment of flying with this Damoclesian sword hanging over one's head, but the fact is that there are several licenced pilots with this potentially disabling problem.
Shortly after my retirement I experienced–for the first time in my life–what was almost certainly A-S. It totally disabled me on the side of the road, with emergency services looking for my car. An English GP later told me the story of leaving an Indian restaurant with his dad. The old guy exploded from both ends and had other symptoms of A-S. His dad was a retired GP with no previous history.
Just worth a mention. I eat peanuts every day, so my reaction is to something never discovered. A chemist guessed that it may have been the plasticiser in the capsules (the plastic coverings) of a tablet that I was taking for my back. However, having kick-started this reaction, I now can not take Pethadine and various other strong pain killers without a very similar reaction.
I am utterly convinced that suggestion played a large part in my change of susceptibility.