Correct Answer: B. Nitrates
Sildenafil is a phosphodiesterase-5 (PDE-5) inhibitor that causes vasodilation through cGMP accumulation. Nitrates (isosorbide dinitrate, isosorbide mononitrate) are organic nitrates that also cause vasodilation via the nitric oxide (NO)–cGMP pathway. When both drugs are used together, there is a synergistic and potentially catastrophic drop in blood pressure—a contraindication recognized in all major guidelines including Indian cardiology practice. The combination can cause severe, refractory hypotension, syncope, myocardial infarction, and death, particularly in patients with underlying coronary artery disease. Sildenafil is absolutely contraindicated in any patient on chronic or acute nitrate therapy. This is a critical drug–drug interaction that must be recognized in clinical practice. Even a single dose of sildenafil within 24–48 hours of nitrate use poses significant risk. In Indian clinical settings, where angina patients are routinely prescribed long-acting nitrates (e.g., isosorbide mononitrate 60 mg daily), this contraindication is paramount and must be counseled to patients presenting with erectile dysfunction.
Why the other options are wrong
A. Calcium channel blockers — Calcium channel blockers (diltiazem, verapamil, amlodipine) cause vasodilation but do NOT share the NO–cGMP mechanism with sildenafil. There is no synergistic hypotensive interaction. Sildenafil can be safely used in patients on CCBs; in fact, CCBs may even be preferred in angina patients with ED because they allow PDE-5 inhibitor use. This is a distractor that tests whether students confuse all vasodilators as contraindicated with sildenafil. C. ACE inhibitors — ACE inhibitors (lisinopril, ramipril, enalapril) lower blood pressure via the renin–angiotensin system, not via cGMP. There is no pharmacological synergy with sildenafil. Sildenafil is safe in ACE inhibitor users and is often used in this population for ED. This option exploits the misconception that any antihypertensive agent contraindicates PDE-5 inhibitors. D. Beta blockers — Beta blockers (metoprolol, atenolol, bisoprolol) reduce cardiac workload and blood pressure via sympathetic blockade, independent of the NO–cGMP pathway. No dangerous interaction exists with sildenafil. Beta blockers are commonly used in angina and do not preclude PDE-5 inhibitor use. This is a common distractor in Indian NEET exams that tests knowledge of the specific nitrate–sildenafil contraindication.
High-Yield Facts
- Sildenafil + nitrates = absolute contraindication due to synergistic cGMP-mediated hypotension and risk of MI/syncope.
- Nitrate washout period: Sildenafil must not be given within 24–48 hours of nitrate use (longer for tadalafil, 72 hours).
- Other PDE-5 inhibitors (tadalafil, vardenafil) are also contraindicated with nitrates via the same mechanism.
- CCBs, ACE-I, and beta-blockers are safe with sildenafil and do not cause dangerous hypotensive interactions.
- Indian guideline: CSSI (Cardiac Society of India and India Heart Rhythm Society) recommends absolute avoidance of PDE-5 inhibitors in nitrate-treated patients.
Mnemonics
NITRATE-SILDENAFIL RULE Nitrates + Sildenafil = NO (contraindicated). Both work via NO–cGMP → synergistic hypotension. SAFE WITH SILDENAFIL CCB-ACE-BB (Calcium Channel Blockers, ACE inhibitors, Beta Blockers) are all SAFE with sildenafil in angina patients.
NBE Trap
NBE pairs "angina patient with ED" to test whether students reflexively think "all antihypertensives are unsafe with sildenafil" rather than recognizing the specific NO–cGMP synergy unique to nitrates. The distractor options (CCB, ACE-I, BB) are all legitimate angina drugs that DO allow sildenafil use.
Clinical Pearl
In Indian outpatient cardiology, many angina patients on long-acting nitrates present with ED and ask about sildenafil. Counseling them on this contraindication and offering alternatives (e.g., switching to a CCB-based regimen if feasible, or using non-pharmacological approaches) is a key safety competency. Always ask about nitrate use before prescribing any PDE-5 inhibitor.
_Reference: KD Tripathi Ch. 31 (Nitrates & Vasodilators); Harrison Ch. 282 (Erectile Dysfunction & Drug Interactions)_