PTCB Guide to PDE5 Inhibitors

Also called phosphodiesterase type 5 inhibitors, the -afil drugs · suffix -afil · updated August 7, 2026

What gets tested

• Recognizing the -afil suffix and matching brand to generic (Viagra and Revatio are sildenafil, Cialis and Adcirca are tadalafil, Levitra is vardenafil)
• The nitrate contraindication, which is the single most important fact about this class
• The alpha-blocker interaction and the blood pressure drop it can cause
• Priapism, meaning an erection lasting more than 4 hours, as a medical emergency
• The duration difference: sildenafil lasts hours while tadalafil can last up to 36
• That the same molecule appears under different brands and strengths for a completely different indication

What is not tested

You are not asked to choose between agents, to dose for renal impairment, or to counsel on sexual health beyond the standard safety points. Concentrate on the suffix, the brand and generic pairs including the dual-indication brands, the nitrate contraindication, and the emergency counseling points. Strengths, dosage forms, routes, special handling, and duration of therapy fall under PTCE knowledge area 1.4, and the strength differences between the ED and pulmonary hypertension products make this a realistic order entry trap.

Why pde5 inhibitors matter

Sildenafil and tadalafil are both heavily dispensed, and the nitrate interaction is one of the few genuinely absolute contraindications a technician will meet at the counter. The dual-brand situation is also a practical safety issue: a patient on Revatio three times daily and a patient on Viagra as needed are taking the same molecule on completely different schedules, and confusing them matters.

PDE5 Inhibitors at a glance

Mechanism of action

Sexual stimulation causes nerve endings and the vessel lining in the penis to release nitric oxide. Nitric oxide activates an enzyme that produces cyclic guanosine monophosphate, or cGMP, and cGMP is the signal that tells smooth muscle in the arteries and the corpus cavernosum to relax. Relaxed smooth muscle lets blood flow in, and the resulting pressure compresses the veins that would drain it, producing an erection. The enzyme phosphodiesterase type 5 breaks cGMP down and ends the signal. PDE5 inhibitors block that enzyme, so cGMP persists and the relaxation is amplified and prolonged. This is why the class does not work without arousal: the drug does not create the nitric oxide signal, it only stops the response being switched off, so some stimulation is still required to start the process. The same enzyme is abundant in the smooth muscle of the pulmonary arteries, which is why sildenafil and tadalafil are also used to lower pulmonary artery pressure in pulmonary arterial hypertension, marketed there as Revatio and Adcirca at different strengths and schedules. The dangerous interaction follows directly from the mechanism. Nitrates such as nitroglycerin work by donating nitric oxide, driving cGMP production up. Adding a PDE5 inhibitor removes the only route for clearing that cGMP, so the vasodilation compounds and blood pressure can collapse. Sildenafil also has weak activity at PDE6 in the retina, which explains the transient blue-tinted vision some patients report.

Common and important side effects

• Headache, facial flushing, and nasal congestion, all from vasodilation
• Dyspepsia and indigestion
• Back pain and muscle aches, reported more with tadalafil
• Visual changes including a blue tint or light sensitivity, most associated with sildenafil
• Priapism, an erection lasting more than 4 hours, which is a medical emergency because it can cause permanent tissue damage
• Non-arteritic anterior ischemic optic neuropathy, a rare cause of sudden vision loss in one eye
• Sudden hearing loss, sometimes with ringing and dizziness
• Dizziness and low blood pressure, particularly alongside other vasodilators

Key interactions

• Nitrates in any form, including nitroglycerin tablets, sprays, patches, and isosorbide, are an absolute contraindication because of the risk of severe hypotension
• Recreational amyl nitrite (poppers) carries the same absolute contraindication
• Alpha-blockers such as doxazosin and tamsulosin can cause a marked blood pressure drop when combined, so doses are separated and started low
• Strong CYP3A4 inhibitors such as ketoconazole, ritonavir, and clarithromycin raise drug levels, so the dose is reduced
• Grapefruit juice can raise levels through the same pathway
• Other antihypertensives add to the blood pressure lowering effect

Patient counseling notes

• Never take this medicine with any nitrate. If chest pain occurs and you need nitroglycerin, tell the emergency team that you have taken a PDE5 inhibitor and when
• Seek emergency care for an erection lasting more than 4 hours, because delay can cause permanent damage
• Sildenafil is usually taken about 30 to 60 minutes before activity, and a high-fat meal can delay its onset
• Tadalafil can remain active for up to 36 hours, so the timing is much more flexible, and it is also available as a low daily dose
• Stop and seek care for sudden vision loss or sudden hearing loss
• Tell the pharmacist about every other medicine, especially anything for the heart, chest pain, or prostate
• The medicine does not work without sexual stimulation, because it amplifies a natural signal rather than creating one

Practice questions

Work the PDE5 Inhibitors questions below first, then widen out. Two skills carry most of the Medications domain, and each has its own quiz. The first is drug class identification: recognising which class a drug belongs to from its name or its stem, which is what the drug class identification quiz drills across every class. The second is brand and generic recall, because the exam switches between the two names freely and expects you to follow. Pair this guide with the brand and generic quiz until the brand and generic pairs in the table above come back without effort.

Related drug-class guides

Official sources

• MedlinePlus: Sildenafil
• MedlinePlus: Tadalafil
• ClinCalc DrugStats: Sildenafil (dispensing data)

PTCB Quiz Prep is an independent study resource, not affiliated with the Pharmacy Technician Certification Board. This guide is for exam preparation, not medical advice. Part of the PTCB Medications domain.