For one, nightmares and hallucinations are not allergic reactions, and should not be described as such. And second, true allergic reactions can vary from a mildly annoying rash to life-threatening anaphylaxis. it's important to know the specifics in order to make a proper risk-benefit analysis.
Because they’re making medical judgements that can kill me if they’re misinformed. If they withhold Tylenol for a fever because “allergic to Vicodin”, and I die from that fever, I will have died because I called it an allergy. It’s bad enough dealing with informed risks without creating uninformed risks too.
In his case, the side effects are hallucinations. Perhaps he's OK with them being prescribed if the alternative is worse.
As a highly intelligent patient, hallucinating nightmares put the hospital risk from me being highly intelligent and hallucinating. This benefits no one to call an allergy.