Final answer:
Idiosyncratic is the term that means an unexpected reaction to a drug.
Step-by-step explanation:
The term that means an unexpected reaction to a drug is idiosyncratic. An idiosyncratic reaction is a unique and individual response to a medication that is not expected based on the normal actions and side effects of the drug. It is unpredictable and can occur even with low doses of the drug. For example, some people may have an idiosyncratic reaction to a painkiller and experience severe allergic reactions instead of pain relief.
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