Cialis (Tadalafil) Complete Drug-Drug Interaction Profile

How Tadalafil Works: The Mechanism Behind Every Interaction

Tadalafil selectively inhibits phosphodiesterase type 5 (PDE5), the enzyme that degrades cyclic guanosine monophosphate (cGMP) in vascular smooth muscle. Blocking PDE5 allows cGMP to accumulate, which activates protein kinase G, reduces intracellular calcium, and produces smooth-muscle relaxation in penile corpora cavernosa as well as the pulmonary vasculature and prostate stroma. That single mechanism is the root cause of nearly every clinically significant drug interaction tadalafil carries.

Why the 17.5-Hour Half-Life Matters

Sildenafil has a half-life of roughly 3 to 5 hours. Vardenafil sits around 4 to 5 hours. Tadalafil's half-life of approximately 17.5 hours means cGMP amplification persists long after the patient feels any subjective effect. FDA prescribing information for tadalafil confirms a time to maximum plasma concentration of 2 hours and a half-life that keeps the drug measurable for up to 5 days. Any co-administered vasodilator administered within that window carries interaction risk.

The CYP3A4 Metabolic Route

Tadalafil is metabolized almost exclusively by hepatic CYP3A4 to an inactive catechol glucuronide. PubMed-indexed pharmacokinetic data confirm that CYP3A4 is the primary elimination pathway. Drugs that inhibit or induce CYP3A4 therefore change tadalafil plasma concentrations substantially, which shifts the magnitude of every pharmacodynamic interaction downstream.

Protein Binding and Volume of Distribution

Tadalafil is approximately 94% plasma-protein-bound, predominantly to albumin and alpha-1-acid glycoprotein. The volume of distribution is roughly 63 L. High protein binding limits displacement interactions in practical clinical use, but it does reinforce that hepatic metabolism, not renal filtration, drives elimination.

Absolute Contraindications: Interactions That Cannot Be Managed

Some drug combinations with tadalafil carry a risk profile that no dose adjustment eliminates.

Nitrates and Nitric Oxide Donors

Organic nitrates, including nitroglycerin, isosorbide mononitrate, isosorbide dinitrate, and amyl nitrite ("poppers"), work by donating nitric oxide, which activates guanylate cyclase and raises cGMP. Tadalafil simultaneously blocks cGMP degradation. The result is an additive, sometimes catastrophic, drop in systemic blood pressure.

The FDA label states that co-administration of tadalafil with any organic nitrate is contraindicated in any form, at any dose, and at any frequency. This language appears in the FDA-approved prescribing information without exception.

Clinicians managing chest pain in a patient who may have taken tadalafil within 48 hours face a specific management problem. The American Heart Association's 2014 update on sexual activity and cardiovascular disease notes that nitrates should be withheld for at least 24 to 48 hours after tadalafil administration, given the drug's long half-life, compared with the shorter 12-hour window recommended after sildenafil or vardenafil. The AHA position statement is available at the American Heart Association journals portal.

Riociguat (Adempas)

Riociguat is a soluble guanylate cyclase stimulator approved for pulmonary hypertension. Like nitrates, it raises cGMP through a different but complementary mechanism. Combining it with tadalafil produces unpredictable, severe hypotension. The FDA label for both agents carries a mutual contraindication.

Dual PDE5 Inhibitor Use

Prescribing tadalafil alongside sildenafil or vardenafil offers no additional efficacy and compounds vasodilatory risk. There is no approved clinical scenario for combining two PDE5 inhibitors.

High-Risk Interactions Requiring Dose Modification

These pairings are not contraindicated outright, but they demand prescriber attention before the patient fills a prescription.

Alpha-Blockers

Alpha-1 adrenergic antagonists (doxazosin, terazosin, prazosin, alfuzosin, tamsulosin, silodosin) are widely prescribed for benign prostatic hyperplasia and hypertension. They lower blood pressure through a different mechanism than tadalafil, but the vasodilatory effects add up.

Brock et al., publishing in the Journal of Urology in 2002, conducted one of the foundational pharmacodynamic studies showing that tadalafil's longer duration distinguished it from sildenafil in both erectile dysfunction and BPH populations, and that alpha-blocker co-administration in BPH patients required specific titration protocols to avoid symptomatic hypotension. The full study is indexed at PubMed.

The FDA label specifically identifies tamsulosin 0.4 mg as the only alpha-blocker dose studied extensively enough with tadalafil 5 mg daily to support a general recommendation. Non-selective alpha-blockers like doxazosin carry a higher hypotension risk in this combination. The recommended strategy is to stabilize the patient on the alpha-blocker first, then introduce tadalafil at the lowest effective dose.

Practical alpha-blocker management framework:

| Alpha-Blocker | Selectivity | Tadalafil Risk Level | Suggested Approach | |---|---|---|---| | Tamsulosin 0.4 mg | Uroselective | Low-moderate | Tadalafil 5 mg daily acceptable after stabilization | | Alfuzosin 10 mg | Uroselective | Moderate | Initiate tadalafil at lowest dose; check standing BP | | Silodosin 8 mg | Uroselective | Moderate-high | Limited combined data; caution advised | | Doxazosin 4-8 mg | Non-selective | High | Avoid or use extreme caution; document BP monitoring | | Terazosin | Non-selective | High | Same caution as doxazosin |

Strong CYP3A4 Inhibitors

Strong CYP3A4 inhibitors substantially increase tadalafil plasma exposure, raising the risk of dose-dependent adverse effects including severe hypotension, priapism, and back/myalgia.

The most clinically relevant agents include:

• Ritonavir and other HIV protease inhibitors. Ritonavir 200 mg twice daily increased tadalafil AUC by 124% in pharmacokinetic studies. The FDA label caps tadalafil at a single 10 mg dose no more than once every 72 hours when co-administered with ritonavir.
• Ketoconazole 400 mg daily. Raises tadalafil AUC by approximately 312% and Cmax by 22%. Ketoconazole interaction data are cited in the FDA drug label. The same 10 mg / 72-hour cap applies.
• Itraconazole and voriconazole. Similar magnitude to ketoconazole; same restriction applies.
• Clarithromycin and erythromycin. Moderate-to-strong CYP3A4 inhibition; reduce tadalafil dose and extend the dosing interval.
• Grapefruit juice. Consistent CYP3A4 inhibition via intestinal furanocoumarins. The effect is variable (20 to 70% AUC increase), but patients on tadalafil should avoid regular grapefruit consumption.

CYP3A4 Inducers

Strong inducers accelerate tadalafil metabolism and may reduce plasma concentrations to sub-therapeutic levels.

Rifampin 600 mg daily reduced tadalafil AUC by approximately 88% in a controlled pharmacokinetic study. This datum appears in published pharmacokinetic analyses available through PubMed. Other strong inducers include carbamazepine, phenytoin, phenobarbital, and St. John's Wort. For patients requiring rifampin (for tuberculosis treatment, for example), a different therapeutic approach for erectile dysfunction or BPH may be needed entirely, because the induction magnitude makes standard dosing unreliable.

Antihypertensive Agents

Tadalafil produces modest reductions in blood pressure even without co-administration of antihypertensives. Adding an antihypertensive amplifies this effect.

Calcium Channel Blockers

Amlodipine 5 mg daily combined with tadalafil 20 mg produced an additional mean reduction of 8 mmHg systolic and 7 mmHg diastolic compared with amlodipine alone in pharmacodynamic studies. Blood-pressure interaction data are summarized in published clinical pharmacology work accessible through the NIH. This magnitude is clinically meaningful but manageable in most patients. Patients with baseline systolic BP below 100 mmHg or on multiple antihypertensives require more careful evaluation.

ACE Inhibitors and ARBs

No clinically significant pharmacokinetic interaction exists between tadalafil and ACE inhibitors or ARBs. The interaction is purely pharmacodynamic: additive vasodilation. Most patients tolerate this combination at standard doses, but the prescriber should document baseline blood pressure and educate the patient about positional hypotension symptoms.

Beta-Blockers

Beta-blockers do not share a metabolic pathway with tadalafil and carry minimal direct pharmacodynamic overlap. Atenolol 50 mg plus tadalafil 10 mg produced only a 1 mmHg additional drop in systolic blood pressure compared with atenolol alone in controlled studies. This combination is generally safe.

Diuretics

Loop diuretics and thiazides reduce preload and can lower blood pressure modestly. Adding tadalafil in a volume-depleted patient carries symptomatic hypotension risk. The interaction is not a reason to withhold tadalafil, but the prescriber should confirm adequate hydration status and consider whether loop diuretic dosing is appropriate before initiating tadalafil.

Alcohol

Alcohol is a vasodilator. Tadalafil is a vasodilator. Their combination is not contraindicated in moderate amounts, but the FDA label warns that consuming more than 5 units of alcohol with tadalafil increased the rate of orthostatic hypotension, dizziness, and tachycardia in pharmacodynamic studies. The FDA label presents this interaction quantitatively. Five units is approximately equivalent to five standard US drinks (each 14 g ethanol). Patients with cardiovascular risk factors should be counseled to limit alcohol to two or fewer drinks when using tadalafil.

Antifungals Beyond Ketoconazole

Fluconazole is a moderate CYP3A4 inhibitor and also inhibits CYP2C9. Its effect on tadalafil is less pronounced than ketoconazole but still clinically relevant in patients on daily tadalafil 5 mg for BPH, where even a 30 to 50% AUC increase shifts the patient toward a higher-exposure range. Fluconazole courses for vaginal candidiasis (single 150 mg dose) carry minimal interaction risk. Prolonged courses (7 to 14 days) at 200 mg daily warrant monitoring.

Antiretrovirals Beyond Ritonavir

Integrase Strand Transfer Inhibitors

Dolutegravir, bictegravir, and raltegravir are not CYP3A4 inhibitors and carry no meaningful pharmacokinetic interaction with tadalafil. They are the preferred ART backbone in patients requiring concurrent tadalafil use.

Non-Nucleoside Reverse Transcriptase Inhibitors

Efavirenz and nevirapine are CYP3A4 inducers. Efavirenz in particular may reduce tadalafil AUC by 40 to 50%, potentially rendering standard doses less effective. Etravirine induces CYP3A4 to a lesser degree. Prescribers managing HIV-positive patients on efavirenz who require tadalafil may need to use the 20 mg on-demand dose rather than 10 mg, while remaining alert to concurrent protease inhibitor exposure if the regimen includes pharmacokinetic boosting.

Drug Interactions in the Context of Pulmonary Arterial Hypertension

Tadalafil 40 mg daily (brand name Adcirca, generically available) is FDA-approved for pulmonary arterial hypertension (PAH). The PAH population introduces additional interaction considerations.

Endothelin receptor antagonists (ERAs) like bosentan are CYP3A4 inducers. Bosentan reduced tadalafil AUC by approximately 42% in a published pharmacokinetic study, which is the basis for the FDA-approved dose for PAH being 40 mg (not 20 mg) when these drugs are co-administered. This pharmacokinetic interaction is indexed in the NIH databases.

Ambrisentan is not a meaningful CYP3A4 inducer, and no dose adjustment is required when combining it with tadalafil for PAH.

Prostacyclin analogues (epoprostenol, treprostinil, iloprost) have additive vasodilatory effects. These combinations are used intentionally in PAH combination therapy and are generally managed under specialist care with invasive hemodynamic monitoring at initiation.

Renal and Hepatic Impairment: How Comorbidities Shift Interaction Severity

Renal impairment does not substantially affect tadalafil pharmacokinetics because the drug is eliminated hepatically. However, the tadalafil label recommends a maximum dose of 10 mg (not more than once every 48 hours) in patients with creatinine clearance 30 to 50 mL/min, and avoidance of tadalafil in patients with creatinine clearance below 30 mL/min who are not on dialysis. Renal dosing guidance appears in the FDA label.

Hepatic impairment directly affects CYP3A4 activity and therefore amplifies every CYP3A4-mediated interaction. Child-Pugh Class A patients may use tadalafil up to 10 mg. Class B hepatic impairment warrants dose reduction and caution. Class C is a contraindication.

The Endocrine Society's clinical practice guideline on testosterone therapy (which frequently accompanies PDE5 inhibitor prescribing) states: "Evaluate cardiovascular risk, medication interactions, and organ function before initiating any vasoactive therapy." The Endocrine Society guideline is accessible at the Endocrine Society's publication portal.

Drug-Drug Interactions Summary Table

| Drug / Drug Class | Interaction Type | Clinical Consequence | Management | |---|---|---|---| | Organic nitrates (all forms) | Pharmacodynamic | Potentially fatal hypotension | Absolute contraindication | | Riociguat | Pharmacodynamic | Severe hypotension | Absolute contraindication | | Ritonavir / strong PI boosters | PK (CYP3A4 inhibition) | Tadalafil AUC up 124% | Max 10 mg / 72 h | | Ketoconazole 400 mg | PK (CYP3A4 inhibition) | Tadalafil AUC up 312% | Max 10 mg / 72 h | | Rifampin 600 mg | PK (CYP3A4 induction) | Tadalafil AUC down ~88% | Consider alternative therapy | | Efavirenz | PK (CYP3A4 induction) | Tadalafil AUC down ~40-50% | May require higher dose | | Bosentan | PK (CYP3A4 induction) | Tadalafil AUC down ~42% | 40 mg PAH dose addresses this | | Non-selective alpha-blockers | Pharmacodynamic | Symptomatic hypotension | Stabilize on alpha-blocker first; start low | | Tamsulosin 0.4 mg | Pharmacodynamic | Modest BP reduction | 5 mg daily generally safe | | Amlodipine | Pharmacodynamic | +8/7 mmHg additional BP drop | Acceptable; monitor baseline BP | | Alcohol (>5 units) | Pharmacodynamic | Orthostatic hypotension, dizziness | Limit to <2 drinks | | Grapefruit juice | PK (intestinal CYP3A4) | AUC up 20-70% | Avoid regular consumption | | Sildenafil / vardenafil | Pharmacodynamic | Additive vasodilation, no added benefit | Never combine |

What Prescribers Often Miss: Less-Obvious Interactions

Most clinicians know the nitrate rule. Several interactions receive less attention in routine practice.

Antiepileptics as inducers. Carbamazepine, phenytoin, and phenobarbital are all moderate-to-strong CYP3A4 inducers. A patient on carbamazepine 600 mg daily for trigeminal neuralgia who is prescribed tadalafil 10 mg on-demand may experience substantially reduced efficacy rather than a drug interaction adverse event. The prescriber may attribute treatment failure to the condition rather than the pharmacokinetic interaction.

Moderate CYP3A4 inhibitors in polypharmacy. Diltiazem, verapamil, and fluconazole each produce 30 to 50% increases in tadalafil AUC in isolation. A patient on all three (not uncommon in older adults with atrial fibrillation and recurrent candidiasis) faces a cumulative CYP3A4 inhibition burden that may require tadalafil dose reduction to 5 mg or below.

Herbal products. St. John's Wort (hypericum perforatum) is a potent CYP3A4 inducer. Patients using it for mild depression may not disclose it as a "medication." Published data on St. John's Wort CYP3A4 induction magnitude are available through NIH resources.