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FDA-approved
Sildenafil (Viagra) was the first PDE5 inhibitor approved by the FDA (1998), for erectile dysfunction — same drug class and mechanism as tadalafil (cGMP breakdown inhibition, promoting penile blood flow), but with a much shorter duration of action (~4 hours vs. tadalafil's ~17.5).
Side Effects
Same core safety profile as the rest of this drug class: headache, flushing, visual disturbances (a PDE5-inhibitor-class effect), and a dangerous interaction with nitrate medications that makes combining the two contraindicated.
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