idiosyncratic reaction
noun1
Medicine, Sciences A reaction to a medication that is unusual and unpredictable, specific to a particular person. Unlike allergy, it can occur on first exposure to the medication; unlike a side effect, it affects only very few individuals.
- The conclusion was that Mr Shakoor had experienced a rare and unpredictable idiosyncratic reaction to them.2001, Margaret Fordham, “The Standard of Care Applicable to Practitioners of Alternative Medicine: Shakoor v Situ”, in Singapore Journal of Legal Studies, page 4: