Examples of antagonists include B1-adrenergic and angiotensin AT1 antagonists which are used to treat high blood pressure. Histamine H1 antagonists are used to treat allergies. Examples of agonists include oxycodone and hydrocodone. These are used to treat pain. Brimonidine is another agonist and it is used to treat glaucoma. G-Protein Coupled Receptors (GPCRs) agonists and antagonists are significant because of the many approved drugs existing in the form of GPCRs, which play a large role in patient care.