antihistaminic

adjective
  1. acting against or counteracting the effects of histamine; used to treat allergies and allergic reactions
  2. Usage: medical/pharmaceutical term; often used in medical contexts
noun
  1. a drug or substance that counteracts the effects of histamine, used to relieve allergy symptoms
  2. Usage: medical/pharmaceutical term; often used interchangeably with 'antihistamine'

Examples