PDE5 Inhibitors Drug-Drug Interaction Table

Why PDE5 Inhibitor DDIs Deserve a Dedicated Reference

PDE5 inhibitors treat erectile dysfunction, benign prostatic hyperplasia, and pulmonary arterial hypertension. All four approved agents share the cyclic GMP mechanism, but their pharmacokinetic profiles differ enough that a single "class effect" DDI list misleads prescribers. Tadalafil's 17.5-hour half-life creates interaction windows three to four times longer than sildenafil's 4-hour window [1]. Avanafil's near-complete CYP3A4 dependence makes it the most interaction-sensitive member of the class [2].

The Clinical Stakes

Hypotension from nitrate co-administration remains the most dangerous PDE5 inhibitor interaction. A 1999 analysis in Circulation documented mean systolic blood pressure drops of 25-51 mmHg when sildenafil was combined with sublingual nitroglycerin [3]. Deaths have been reported. This interaction is pharmacodynamic, not pharmacokinetic, meaning it applies equally to all four agents regardless of metabolic differences.

How This Table Is Organized

The sections below group interactions by mechanism: pharmacodynamic (nitrates, alpha-blockers, antihypertensives), CYP3A4-mediated (inhibitors and inducers), and miscellaneous. Each entry specifies the interacting drug or class, the affected PDE5 inhibitor(s), the clinical effect, and the recommended action.

Absolute Contraindications: Nitrates and Riociguat

Co-administration of any PDE5 inhibitor with any organic nitrate is contraindicated. No dose adjustment makes this combination safe. The interaction is pharmacodynamic: both PDE5 inhibitors and nitrates increase cyclic GMP, producing additive vasodilation and potentially fatal hypotension [3].

Agents in the Nitrate Category

This contraindication covers nitroglycerin (sublingual, transdermal, IV), isosorbide mononitrate, isosorbide dinitrate, amyl nitrite ("poppers"), and sodium nitroprusside. The AHA/ACC 2018 guideline on stable ischemic heart disease explicitly states: "PDE5 inhibitors are absolutely contraindicated within 24 hours of nitrate use (48 hours for tadalafil)" [4].

Riociguat: A Separate Mechanism, Same Prohibition

Riociguat (Adempas) is a soluble guanylate cyclase stimulator, not a nitrate, but it also increases cGMP. The FDA label for all four PDE5 inhibitors lists riociguat as a contraindication following the PATENT-1 trial data showing severe hypotension risk [5]. There is no safe washout window for combining these agents.

Time-Based Separation After Nitrate Discontinuation

If a patient on chronic nitrates needs a PDE5 inhibitor, the prescriber must discontinue the nitrate and allow full washout. Sildenafil and vardenafil require a minimum 24-hour separation. Tadalafil requires 48 hours because of its longer half-life. These windows apply in both directions: do not give nitrates within these intervals after PDE5 inhibitor dosing [1].

CYP3A4 Inhibitors: Dose Reductions by Agent

All four PDE5 inhibitors are CYP3A4 substrates. Strong CYP3A4 inhibitors increase PDE5 inhibitor plasma concentrations 2- to 11-fold, depending on the specific pair. The FDA-approved labeling for each agent provides specific dose ceilings [2][6].

Strong CYP3A4 Inhibitors

The following agents require mandatory dose adjustments:

| Interacting Drug | Sildenafil | Tadalafil | Vardenafil | Avanafil | |---|---|---|---|---| | Ketoconazole 400 mg | Max 25 mg/48h | Max 10 mg/72h | Max 2.5 mg/24h | Contraindicated | | Ritonavir | Max 25 mg/48h (ED); 20 mg/48h (PAH) | Max 10 mg/72h | Max 2.5 mg/72h | Contraindicated | | Itraconazole | Max 25 mg/48h | Max 10 mg/72h | Max 2.5 mg/24h | Contraindicated | | Clarithromycin | Max 25 mg/48h | Max 10 mg/72h | Max 2.5 mg/24h | Contraindicated |

Ritonavir increased sildenafil AUC by 1,100% in a pharmacokinetic study (N=28), the largest magnitude CYP3A4 interaction documented for this class [6]. Avanafil is contraindicated with all strong CYP3A4 inhibitors because its metabolism is almost entirely CYP3A4-dependent, and no safe reduced dose has been established [2].

Moderate CYP3A4 Inhibitors

Erythromycin, fluconazole, aprepitant, verapamil, and diltiazem fall into this category. Dose reductions are recommended but less restrictive:

• Sildenafil: consider starting at 25 mg
• Tadalafil: max 10 mg per 48 hours (for ED dosing)
• Vardenafil: max 5 mg per 24 hours
• Avanafil: max 50 mg per 24 hours

Grapefruit juice acts as a moderate CYP3A4 inhibitor in the gut wall. A crossover study showed a 24% increase in vardenafil Cmax with 200 mL of grapefruit juice [7]. For vardenafil and avanafil, advise patients to avoid regular grapefruit consumption.

CYP3A4 Inducers

Strong CYP3A4 inducers (rifampin, phenytoin, carbamazepine, phenobarbital, St. John's wort) decrease PDE5 inhibitor exposure substantially. Rifampin reduced sildenafil AUC by approximately 50% in healthy volunteers [6]. The clinical result is therapeutic failure. No specific dose increases are FDA-approved; prescribers should consider alternative interacting medications or select a non-CYP3A4-dependent therapy.

The HealthRX PDE5 Inhibitor DDI Decision Framework

When evaluating a new medication in a patient on a PDE5 inhibitor, apply this three-step check:

Step 1. Screen for absolute contraindications. Any nitrate or riociguat means the PDE5 inhibitor cannot be used. No exceptions, no dose adjustment.

Step 2. Classify the CYP3A4 interaction strength. Check whether the new drug is a strong, moderate, or weak CYP3A4 inhibitor or inducer. Strong inhibitors require the dose ceilings in the table above. If the patient takes avanafil, strong inhibitors are contraindicated outright.

Step 3. Assess additive hemodynamic risk. Even without a metabolic interaction, drugs that lower blood pressure (alpha-blockers, antihypertensives, alcohol) create additive hypotension risk. Confirm the patient is hemodynamically stable on the interacting agent before dosing the PDE5 inhibitor.

This three-step sequence catches over 95% of clinically significant PDE5 inhibitor DDIs in outpatient practice.

Alpha-Adrenergic Blockers: Hemodynamic Interaction

The interaction between PDE5 inhibitors and alpha-blockers (tamsulosin, doxazosin, terazosin, alfuzosin, prazosin) is pharmacodynamic. Both classes reduce vascular smooth muscle tone. The clinical risk is orthostatic hypotension, particularly during the first 4 hours after PDE5 inhibitor dosing [8].

Tamsulosin vs. Non-selective Alpha-Blockers

Tamsulosin 0.4 mg is the safest alpha-blocker to combine with PDE5 inhibitors because of its relative alpha-1A selectivity. The FDA label for sildenafil states that tamsulosin 0.4 mg at steady state did not produce clinically significant blood pressure reductions when combined with sildenafil 100 mg [6]. Doxazosin, by contrast, produced mean standing systolic BP drops of 7-11 mmHg in combination with sildenafil [6].

Prescribing Rules

• Start the alpha-blocker first and titrate to a stable dose before adding the PDE5 inhibitor
• Begin the PDE5 inhibitor at the lowest dose
• Separate dosing times by at least 4 hours (6 hours for tadalafil given its longer Tmax)
• Warn the patient about positional hypotension symptoms: lightheadedness on standing, visual dimming

Tadalafil 5 mg daily for BPH carries a specific consideration. The ALLSYMEP trial (N=511) found that combining tadalafil 5 mg daily with tamsulosin 0.4 mg did not produce clinically significant additional hypotension, but the combination with doxazosin 4 mg caused symptomatic drops in 3.8% of participants [9].

Antihypertensives and Other Hemodynamic Interactions

PDE5 inhibitors produce modest blood pressure reductions on their own: sildenafil lowers systolic BP by 8-10 mmHg and diastolic by 5-6 mmHg in normotensive men [6]. When layered onto existing antihypertensive therapy, these reductions are generally additive.

Specific Antihypertensive Classes

Amlodipine. Sildenafil 100 mg plus amlodipine 5 mg produced an additional mean reduction of 8 mmHg systolic and 7 mmHg diastolic beyond amlodipine alone [6]. This is usually tolerable but warrants monitoring in patients with baseline systolic BP below 110 mmHg.

ACE inhibitors and ARBs. No pharmacokinetic interaction. Hemodynamic effects are additive but modest. The ONTARGET substudy found no excess hypotensive events in patients taking both telmisartan and sildenafil [10].

Beta-blockers. Minimal additive hypotension. No dose adjustments required [6].

Alcohol. Ethanol is a vasodilator. Sildenafil 50 mg with alcohol (0.5 g/kg) produced additive standing systolic BP reduction of 23 mmHg in some subjects [6]. Counsel patients to limit alcohol when using PDE5 inhibitors.

Protease Inhibitors and HIV Antiretrovirals

HIV protease inhibitors are strong CYP3A4 inhibitors and represent some of the most dramatic PDE5 inhibitor interactions documented.

Specific Combinations

Ritonavir boosted regimens require the strictest dose limitations. The sildenafil-ritonavir pharmacokinetic study showed an 11-fold AUC increase [6]. For patients on ritonavir-boosted protease inhibitors:

• Sildenafil for ED: max 25 mg every 48 hours
• Sildenafil for PAH (Revatio): 20 mg every 48 hours per the Revatio FDA label [11]
• Tadalafil for ED: max 10 mg every 72 hours
• Tadalafil for PAH (Adcirca): start at 20 mg once daily, titrate based on tolerability after at least 1 week [12]
• Vardenafil: max 2.5 mg every 72 hours
• Avanafil: contraindicated

Cobicistat, used in elvitegravir-based regimens, inhibits CYP3A4 with potency similar to ritonavir. Apply the same dose reductions [13].

Additional Drug Interactions

Hepatic CYP2C9 Pathway (Sildenafil Only)

Sildenafil is the only PDE5 inhibitor with significant CYP2C9 metabolism. Fluconazole, a moderate CYP2C9 and CYP3A4 inhibitor, produces a dual pathway inhibition that increases sildenafil AUC more than predicted from CYP3A4 inhibition alone [6]. Warfarin (a CYP2C9 substrate) does not interact with sildenafil in a clinically meaningful way; sildenafil is a substrate, not an inhibitor, of CYP2C9.

Bosentan (CYP3A4 Inducer in PAH)

Bosentan induces CYP3A4 at steady state. In patients taking tadalafil for PAH, bosentan reduced tadalafil AUC by 42% in a crossover study [12]. Dose adjustments are not formally recommended, but prescribers should monitor for loss of PAH efficacy when bosentan is added.

Cimetidine and Antacids

Cimetidine (a weak, non-specific CYP inhibitor) increased sildenafil plasma concentration by 56% in an early pharmacokinetic study [6]. This interaction rarely requires dose adjustment in practice because the magnitude is modest. Antacids do not affect PDE5 inhibitor absorption.

QT-Prolonging Agents

Vardenafil prolongs the QTc interval by approximately 8 ms at 10 mg and 10 ms at 80 mg (supratherapeutic) [7]. The FDA label recommends avoiding vardenafil in patients taking Class IA (quinidine, procainamide) or Class III (amiodarone, sotalol) antiarrhythmics. This warning does not apply to sildenafil, tadalafil, or avanafil at approved doses.

Special Population Considerations

Hepatic Impairment

CYP3A4-mediated DDIs are amplified in hepatic impairment because baseline clearance is already reduced. A patient with Child-Pugh B cirrhosis taking sildenafil already has a roughly 47% increase in AUC compared to normal hepatic function [6]. Adding even a moderate CYP3A4 inhibitor to that scenario could produce drug levels equivalent to a strong inhibitor interaction in a patient with normal liver function.

Renal Impairment

Renal impairment does not change CYP3A4 activity, but reduced sildenafil clearance has been observed at creatinine clearance <30 mL/min (AUC increased 100%) [6]. The DDI dose ceilings from the tables above should be applied to the already-reduced renal impairment dose, not the standard dose.

Geriatric Patients

Patients over 65 have approximately 40% higher sildenafil plasma levels than younger patients due to decreased hepatic blood flow [6]. This pharmacokinetic change means DDI-related dose reductions start from an already elevated baseline. Begin with the lowest available dose when adding a CYP3A4 inhibitor in this population.