Phenylephrine is a neurotransmitter released by the parasympathetic nervous system, primarily involved in the regulation of heart rate and digestion.

Phenylephrine is a hormone secreted by the adrenal cortex, primarily responsible for the regulation of blood glucose levels and metabolism.

Phenylephrine is a sympathomimetic drug that acts as an alpha-adrenergic agonist, causing vasoconstriction and increasing blood pressure.

Phenylephrine is a hormone produced by the posterior pituitary gland, responsible for water retention and blood pressure regulation.