Tadalafil is a phosphodiesterase type 5 (PDE-5) inhibitor developed for the treatment of erectile dysfunction (ED) and pulmonary arterial hypertension (PAH). It acts by relaxing vascular smooth muscle and enhancing blood flow through nitric oxide (NO)–cGMP signaling. Compared to similar agents such as sildenafil, tadalafil has a significantly longer half-life, providing sustained vasodilatory effects for up to 36 hours. In physiology, it is also studied for its influence on vascular function and oxygen delivery, although non-medical use is not recommended. Main Effects Improves vascular dilation and blood circulation; Supports pulmonary blood flow and oxygenation; May reduce muscular fatigue via improved tissue perfusion; Enhances nitric oxide signaling; Decreases pulmonary vascular resistance; Promotes smooth muscle relaxation and cardiovascular efficiency Precautions Avoid combination with nitrate medications or nitric oxide donors; Monitor blood pressure due to vasodilatory effects; Use cautiously in patients with cardiovascular conditions; Not intended for performance enhancement use Side Effects Headache, flushing, nasal congestion; Indigestion or gastrointestinal discomfort; Back pain or muscle aches; Dizziness due to lowered blood pressure; Rare visual disturbances Contraindications Use with nitrates or NO donors; Severe cardiovascular disease or recent myocardial infarction/stroke; Uncontrolled hypertension or hypotension; Severe liver or kidney impairment; Hypersensitivity to tadalafil Storage Store at 20–25 °C (68–77 °F); Keep in a dry, dark place; Protect from moisture and sunlight; Keep out of reach of children Mechanism of Action Tadalafil selectively inhibits phosphodiesterase type 5 (PDE-5), the enzyme responsible for the degradation of cyclic guanosine monophosphate (cGMP). By preventing cGMP breakdown, it prolongs smooth muscle relaxation and enhances blood flow in vascular tissues.