Prostacyclin is a member of the family of lipid molecules known as eicosanoids. It is produced in endothelial cells from prostaglandin H₂ (PGH₂) by the action of the enzyme prostacyclin synthase . Although prostacyclin is considered an independent mediator, it is called PGI₂ (prostaglandin I₂) in eicosanoid nomenclature, and is a member of the prostanoids (together with the prostaglandins and thromboxane).

Prostacyclin acts chiefly to prevent platelet formation and clumping involved in blood clotting. It is also an effective vasodilator. Prostacyclin's interactions in contrast to thromboxane, another eicosanoid, strongly suggest a mechanism of cardiovascular homeostasis between the two hormones in relation to vascular damage.

Synthetic prostacyclin analogues (iloprost, cisaprost) are used:

• as a vasodilator in severe Raynaud's phenomenon or ischemia of a limb;
• in pulmonary hypertension.

it is inhibited indirectly by aspirin (via cyclooxygenase COX).