I just missed a question on this topic in the USMLE World QBank.
If a patient has taken sildenafil, at least 6 hours should pass before taking nitrates. The sildenafil, obviously, is used to treat erectile dysfunction or pulmonary arterial hypertension, while nitrates are used to treat angina or pulmonary edema. When used during an MI, nitrates can decrease the size of the infarct and decrease mortality.
Why should sildenafil and nitrates not be combined? Because they affect the same NO/cGMP pathway and have additive effects which can cause severe hypotention and possibly an MI.
Sildenafil MOA: It is a PDE5 (cGMP phosphodiesterase 5) inhibitor, so it prevents the degradation of cGMP by PDE5. More cGMP results in vasodilation, and thus more blood flow to the penis.
Nitrates MOA: Examples of nitrates include nitroglycerin and isosorbide dinitrate. They release NO in smooth muscles, thus activating the cGMP pathway. NO activates guanylate cyclase, which makes cGMP. cGMP then causes the dephosphorylation myosin light chain phosphate so that it can no longer interact with actin. This results in smooth muscle relaxation and vasodilation (veins more than arteries).
When I read about the drug interaction, it did sound a wee bit familiar. Here is a clip from "Something's Gotta Give" to help you remember this drug interaction and a potential case presentation.
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