过敏 (guòmǐn) has two meanings: a medical allergy (to substances like peanuts) and a figurative oversensitivity (to words or actions). 多心 (duōxīn) means 'oversensitive' only in the sense of being overly suspicious or taking things personally — it never refers to physical allergies. Use 过敏 for both biological reactions and emotional touchiness; use 多心 when someone misinterprets harmless remarks as personal attacks.