Least predictable reaction to drug:

Unpredictable (Type B or Bizarre) reactions These are based on peculiarities of the patient and not on drug's known actions; include allergy and idiosyncrasy. They are less common, often non-dose related, generally more serious and require withdrawal of the drug. Some of these reactions can be predicted and prevented if their genetic basis is known and suitable test to characterize the individual’s phenotype is performed. Reference: Essentials of Medical Pharmacology Eighth Edition KD TRIPATHI page no 92