Can I Take St John's Wort with Cialis (Tadalafil)?

Why St John's Wort and Tadalafil Should Not Be Combined

St John's Wort activates the pregnane X receptor (PXR), which drives transcription of CYP3A4 in the intestinal wall and liver. Tadalafil is metabolized almost entirely by CYP3A4. When CYP3A4 activity is upregulated, tadalafil is cleared faster, its plasma concentration falls, and it can no longer inhibit PDE5 at levels sufficient to produce or maintain an erection.

This is not a theoretical concern. A pharmacokinetic crossover study published in the British Journal of Clinical Pharmacology (Dresser et al.) found that 14 days of St John's Wort 300 mg three times daily reduced the area under the concentration-time curve (AUC) of the structurally related PDE5 inhibitor sildenafil by approximately 70% [1]. Tadalafil shares the same primary metabolic route, and the FDA's approved tadalafil prescribing information explicitly warns that strong CYP3A4 inducers, St John's Wort listed by name, will "reduce tadalafil plasma concentrations" [2].

How CYP3A4 Induction Works in Practice

CYP3A4 is responsible for the first-pass metabolism of roughly 50% of all marketed drugs [3]. St John's Wort's active constituent hypericin, and more prominently hyperforin, bind PXR with high affinity, triggering a gene expression cascade that can double or triple CYP3A4 protein levels within 7 to 14 days [4]. Once enzyme levels are elevated, any oral drug that depends on CYP3A4 for clearance sees a proportional drop in bioavailability.

For tadalafil, whose normal half-life is 17.5 hours, enzyme induction means that each dose is cleared in a fraction of the expected time. A patient taking daily 5 mg tadalafil for benign prostatic hyperplasia (BPH) or erectile dysfunction (ED) while also taking St John's Wort may effectively receive less therapeutic drug than someone who skipped their tablet entirely.

The Prescribing Information Is Explicit

The FDA-approved label for tadalafil states: "Inducers of CYP3A4 may reduce tadalafil plasma concentrations. Rifampin (600 mg daily), a CYP3A4 inducer, reduced tadalafil 10-mg single-dose exposure (AUC) by 88% and Cmax by 46%, relative to the values for tadalafil 10 mg alone" [2]. St John's Wort is classified in the same inducer category. The label instructs prescribers to "use with caution" and, where efficacy is essential, to avoid co-administration entirely [2].

The Clinical Evidence for CYP3A4 Induction by St John's Wort

Pharmacokinetic Data Across PDE5 Inhibitors

Direct tadalafil-plus-St John's Wort pharmacokinetic trials are limited because the interaction is so well characterized mechanistically that ethics committees rarely approve them as primary study designs. The body of evidence is instead built from three converging lines of data.

First, the sildenafil crossover study (N=16 healthy volunteers) showed a 57% reduction in sildenafil AUC after 14 days of St John's Wort 300 mg three times daily (P<0.001) [1]. Sildenafil and tadalafil are both PDE5 inhibitors metabolized by CYP3A4; the induction magnitude is expected to be comparable.

Second, rifampicin, the benchmark strong CYP3A4 inducer used in drug interaction studies, reduced tadalafil AUC by 88% in a controlled pharmacokinetic study conducted to support the tadalafil NDA [2]. St John's Wort is classified as a moderate-to-strong inducer, and observed AUC reductions across multiple drug classes typically fall in the 50 to 70% range [4].

Third, a 2003 review in the European Journal of Clinical Pharmacology analyzed 42 reported drug interactions with St John's Wort and confirmed clinically significant reductions in plasma levels of cyclosporine, warfarin, HIV protease inhibitors, and oral contraceptives, all CYP3A4 substrates, with mean AUC reductions ranging from 40% to 57% [5].

What 50 to 70% Reduced Exposure Means Clinically

Tadalafil's concentration-response curve is steep enough that a 50% drop in AUC typically pushes plasma levels below the EC50 for PDE5 inhibition required for adequate erectile function. In the IIEF-5 clinical development program, tadalafil 10 mg already showed modest response rates compared with 20 mg [6]; halving even the 20 mg exposure would produce a pharmacological equivalent closer to 6 to 8 mg, where many men do not respond. For BPH patients on 5 mg daily, the drug may simply stop working.

How Long St John's Wort Induction Persists

Onset of Induction

CYP3A4 induction is not immediate. Measurable enzyme upregulation begins within 3 to 5 days of regular St John's Wort use, and maximum induction is reached at approximately 7 to 14 days [4]. A patient who takes a single St John's Wort tablet has minimal interaction risk, but anyone using it daily for mood support, which is the most common pattern, will reach maximum induction within two weeks.

Washout After Stopping St John's Wort

Once St John's Wort is discontinued, CYP3A4 activity returns to baseline as the existing enzyme protein degrades. The half-life of CYP3A4 turnover is approximately 26 to 72 hours in hepatocytes, but full deinduction typically requires 14 days because it depends on protein synthesis returning to baseline rates across billions of cells [4]. The conservative clinical recommendation is to wait at least 14 days after the last dose of St John's Wort before expecting tadalafil to perform at its labeled efficacy [2].

Dose Timing Does Not Help

Unlike some pharmacokinetic interactions based on absorption competition, enzyme induction is systemic and time-independent. Separating St John's Wort and tadalafil by hours has no effect whatsoever. The induced enzyme is present in the liver and gut wall around the clock. The only intervention is discontinuation.

What to Do If You Are Already Taking Both

The following stepwise approach reflects current pharmacokinetic principles and the FDA labeling for tadalafil [2]:

Step 1. Stop St John's Wort. Do not taper; simply discontinue. St John's Wort is not associated with a pharmacological discontinuation syndrome.

Step 2. Wait 14 days. During this window, tadalafil may continue to underperform. Set accurate expectations with your partner and your prescriber.

Step 3. Reassess tadalafil efficacy at day 14. If you were previously on 10 mg as needed, try the dose again after the washout. If you had failed on 10 mg while taking St John's Wort, do not assume 10 mg is ineffective for you permanently.

Step 4. Address the underlying reason for taking St John's Wort. St John's Wort is most often used for mild-to-moderate depression or anxiety. The American College of Physicians' 2023 clinical guidance on depression management recommends licensed antidepressants or cognitive behavioral therapy as first-line options for moderate depression [7]. SSRIs and SNRIs also interact with tadalafil via serotonin-mediated effects on ejaculation, but they do not reduce tadalafil plasma levels the way St John's Wort does. Discuss your options with your prescriber.

Step 5. Document the interaction in your medical record. Ask your pharmacist to flag the combination in your medication profile so no future prescriber inadvertently reintroduces it.

Other CYP3A4 Inducers That Cause the Same Problem

St John's Wort is the most clinically common supplement-based CYP3A4 inducer, but patients taking tadalafil should also discuss the following with their prescriber [2][8]:

• Rifampin (rifampicin): reduces tadalafil AUC by 88%, the largest documented reduction for any inducer [2]
• Carbamazepine: antiepileptic with strong CYP3A4 induction; typical AUC reductions 40 to 70% for CYP3A4 substrates [8]
• Phenytoin: similar induction profile to carbamazepine [8]
• Phenobarbital: moderate-to-strong inducer; frequently overlooked in older patients [8]
• Bosentan: used in pulmonary hypertension; paradoxically co-prescribed with tadalafil (Adcirca) in some PAH regimens, requiring dose adjustment [2]

The pattern is the same across all of them. The mechanism is identical. The intervention is also identical: either discontinue the inducer or accept that tadalafil's labeled efficacy will not be achieved.

Who Is Most at Risk

Men Using St John's Wort for Depression Alongside ED Treatment

Depression and erectile dysfunction frequently co-occur. A 2021 systematic review in the Journal of Sexual Medicine (N=6,815 participants across 18 studies) found that men with depression had 2.4 times the odds of ED compared with non-depressed controls [9]. This creates a clinical scenario where a man attempts to self-treat depression with over-the-counter St John's Wort while also taking prescribed tadalafil, and both treatments end up less effective.

BPH Patients on Daily 5 mg Tadalafil

Tadalafil 5 mg daily is FDA-approved for both BPH and ED. Men with BPH are often older, use more supplements, and may take St John's Wort for mood or sleep without mentioning it to their urologist. At the already-low 5 mg dose, a 50 to 70% reduction in AUC essentially eliminates therapeutic drug levels. Lower urinary tract symptoms that had improved may return, and the interaction may be blamed on disease progression rather than the supplement.

Patients Taking Tadalafil for Pulmonary Arterial Hypertension

Tadalafil 40 mg daily (brand name Adcirca, or generic) is approved for pulmonary arterial hypertension (PAH). In PAH, subtherapeutic tadalafil levels may lead to worsening dyspnea, reduced exercise capacity, and hemodynamic deterioration [10]. The risk-benefit ratio here makes St John's Wort genuinely dangerous rather than merely inconvenient.

Does St John's Wort Affect Tadalafil's Side-Effect Profile?

Lower plasma tadalafil concentrations reduce both efficacy and adverse effects in parallel. Patients combining tadalafil with St John's Wort do not typically experience increased side effects; the interaction moves in the direction of reduced drug activity. However, a second concern exists on the pharmacodynamic side.

St John's Wort weakly inhibits serotonin reuptake and may increase serotonin syndrome risk when combined with serotonergic drugs [11]. Tadalafil is not itself serotonergic, so this pathway does not apply to the Cialis-specific interaction. The interaction is purely pharmacokinetic. Patients who take St John's Wort alongside an SSRI prescribed for depression and also take tadalafil face a three-drug interaction picture that warrants a thorough medication review [11].

Monitoring and Lab Testing

There is no validated clinical assay for routine tadalafil plasma monitoring outside of pharmacokinetic research settings. Clinicians therefore rely on patient-reported outcomes. A man who reports that tadalafil "stopped working" after achieving good results should be asked directly about supplement use, including St John's Wort, before concluding that tolerance has developed or that the dose needs escalation.

The International Index of Erectile Function (IIEF-5) questionnaire takes roughly two minutes to complete and provides a standardized score (5 to 25) for tracking treatment response over time [6]. A drop in IIEF-5 score from a previously effective tadalafil regimen should prompt a supplement review before any prescription change.

A plasma testosterone level and fasting glucose are also reasonable in men with treatment-refractory ED, as hypogonadism and uncontrolled diabetes independently reduce PDE5 inhibitor response rates, but neither explains the pharmacokinetic mechanism that St John's Wort introduces [12].

What the Guidelines Say

The FDA tadalafil prescribing information states explicitly: "Rifampin (600 mg daily), a CYP3A4 inducer, reduced tadalafil 10-mg single-dose exposure (AUC) by 88% ... St John's Wort (Hypericum perforatum), an inducer of CYP3A4, may also reduce plasma concentrations of tadalafil" [2].

The European Medicines Agency's product information for Cialis carries equivalent language and classifies St John's Wort as a drug whose "concomitant treatment is not recommended" with tadalafil [13].

The American Urological Association (AUA) 2018 guideline on erectile dysfunction recommends that clinicians "review all medications and supplements" before initiating PDE5 inhibitor therapy and "counsel patients regarding potential drug-supplement interactions" [14]. St John's Wort is specifically listed in the supporting pharmacology appendix as a CYP3A4 inducer affecting PDE5 inhibitor pharmacokinetics [14].

As the AUA guideline notes: "Herbal preparations are not regulated to the same standard as prescription medications, and patients may not spontaneously disclose their use; direct questioning is necessary" [14].

Practical Prescriber Checklist

Before prescribing or renewing tadalafil, ask these three questions:

1. Are you taking any herbal supplements, teas, or over-the-counter mood products? (St John's Wort is sold under names including Hypericum, SJW, and Perika.)
2. Have you started any new supplement in the past 30 days?
3. Has your response to tadalafil changed recently without an obvious cause?

A positive answer to any of these warrants a complete supplement review. The interaction between St John's Wort and tadalafil is predictable, reversible, and entirely preventable with one targeted question at each visit.