Can I Take Glycine with Cialis (Tadalafil)?

What Is Tadalafil and How Does It Work?

Tadalafil is a phosphodiesterase-5 (PDE5) inhibitor approved by the FDA for erectile dysfunction, benign prostatic hyperplasia, and pulmonary arterial hypertension. It raises cyclic GMP levels in vascular smooth muscle, producing vasodilation. The drug reaches peak plasma concentration in about 2 hours and has a half-life of approximately 17.5 hours, which explains its 36-hour window of action compared with sildenafil's 4-to-6-hour window [1].

CYP3A4 Metabolism

Tadalafil is almost entirely metabolized by hepatic CYP3A4 [1]. Any compound that inhibits or induces this enzyme can raise or lower tadalafil plasma levels significantly. Strong CYP3A4 inhibitors such as ketoconazole increase tadalafil AUC by up to 312%, while rifampin (a strong inducer) reduces tadalafil AUC by 88% [1]. This enzyme pathway is the first place to check when evaluating any supplement taken alongside tadalafil.

Approved Doses

The FDA-approved dosing for tadalafil ranges from 2.5 mg daily (BPH or ED maintenance) to 20 mg on demand (ED). The 5 mg once-daily tablet is the most common starting dose for BPH and as-needed ED prophylaxis [1]. Pulmonary arterial hypertension dosing reaches 40 mg daily under the brand Adcirca, a different clinical population entirely.

What Is Glycine and Why Do People Take It?

Glycine is the simplest amino acid, biosynthesized endogenously and consumed through dietary protein. Supplement users take it for three main reasons: sleep quality, collagen support, and metabolic health. Each use case has a distinct evidence base, and each has a different theoretical relevance to tadalafil.

Sleep and Neuroinhibition

As an inhibitory neurotransmitter in the brainstem and spinal cord, glycine activates strychnine-sensitive receptors that lower core body temperature and promote sleep onset [2]. A randomized crossover trial (N=11) published in Sleep and Biological Rhythms found that 3 g glycine taken before bed reduced subjective fatigue scores and improved polysomnographic sleep quality without sedative side effects [2]. Tadalafil does not affect sleep architecture, so this use case creates no pharmacodynamic overlap.

Collagen Synthesis

Glycine makes up roughly one-third of collagen's amino acid composition [3]. Supplementation at doses between 5 g and 10 g daily is used to support connective tissue repair. A 2019 study in Nutrients (N=102) found that 5 g/day of glycine combined with hydrolyzed collagen improved skin elasticity over 12 weeks compared with placebo [3]. Tadalafil has no known effect on collagen metabolism.

Glycemic and Metabolic Effects

Glycine may improve insulin sensitivity by enhancing glucagon-like peptide-1 (GLP-1) secretion and reducing pro-inflammatory cytokines [4]. A trial published in PLOS ONE found lower fasting plasma glycine levels in men with type 2 diabetes compared with normoglycemic controls, and glycine supplementation modestly reduced fasting glucose over 3 months [4]. Men with metabolic syndrome often take tadalafil for ED (a common comorbidity), so this metabolic effect is worth acknowledging even if it does not create a direct drug interaction.

Does Glycine Interact With Tadalafil Pharmacokinetically?

No pharmacokinetic interaction between glycine and tadalafil has been identified in the published literature. Glycine is not a substrate, inhibitor, or inducer of CYP3A4, CYP2C9, or any other cytochrome P450 enzyme involved in tadalafil's metabolism [5]. It is also not a P-glycoprotein inhibitor, meaning it does not alter tadalafil's absorption or efflux from enterocytes.

Absorption

Tadalafil bioavailability is approximately 80% and is unaffected by food. Glycine, taken as a powder or capsule, is absorbed through small-intestinal amino acid transporters (specifically the glycine transporter GlyT-1 and neutral amino acid transporter systems) [5]. These transporters do not interact with tadalafil's lipophilic absorption pathway. Timing of administration relative to meals does not require adjustment when both compounds are used together.

Protein Binding

Tadalafil is 94% protein-bound, primarily to albumin [1]. Glycine at supplemental doses does not displace tadalafil from albumin binding sites. Free amino acids compete for specific carrier proteins (alpha-1-acid glycoprotein transporters), not for the albumin sites relevant to tadalafil binding.

Renal and Hepatic Clearance

Tadalafil metabolites are excreted 61% in feces and 36% in urine [1]. Glycine is filtered freely at the glomerulus and reabsorbed via renal amino acid transporters. No shared renal clearance pathway exists between the two compounds.

Does Glycine Interact With Tadalafil Pharmacodynamically?

This is where modest caution is warranted. Both glycine and tadalafil can lower blood pressure, though through separate mechanisms. Tadalafil raises cGMP in vascular smooth muscle and reduces systemic vascular resistance [1]. Glycine may reduce blood pressure through nitric oxide modulation and central nervous system inhibition [6].

Blood Pressure Overlap

A meta-analysis of amino acid supplementation trials published in the American Journal of Clinical Nutrition found that glycine supplementation produced a mean reduction in systolic blood pressure of approximately 2 to 4 mmHg in subjects with elevated baseline blood pressure [6]. Tadalafil alone produces a mean 8 to 10 mmHg reduction in systolic blood pressure in hypertensive men [1]. At typical supplement doses (3 to 5 g glycine), the additive hypotensive effect is expected to be small, roughly 2 to 4 mmHg combined.

When Additive Hypotension Becomes Clinically Relevant

The combination becomes more clinically significant in three specific situations. First, patients who also take alpha-blockers (tamsulosin, alfuzosin) for BPH are already at risk for orthostatic hypotension when tadalafil is added, and glycine adds a small further increment [7]. Second, patients taking antihypertensive medications including ACE inhibitors or calcium channel blockers may see a more pronounced combined blood pressure drop. Third, men with baseline systolic blood pressure <100 mmHg should be monitored more carefully.

Nitric Oxide Pathway Interaction

Glycine serves as a precursor to glutathione and influences endothelial nitric oxide synthase (eNOS) activity [6]. Tadalafil's downstream mechanism depends on nitric oxide signaling, specifically the NO-cGMP cascade. Theoretically, higher glycine availability could augment NO bioavailability and modestly potentiate tadalafil's vasodilatory effect. This mechanism has not been tested directly in a clinical trial, but the biochemical rationale supports monitoring blood pressure rather than avoiding the combination.

Glycine's Effect on Sleep and Tadalafil's Nocturnal Erections

Men taking tadalafil daily for ED or BPH sometimes notice nocturnal penile tumescence (NPT) as part of the drug's sustained action. Glycine's sleep-promoting properties may interact indirectly with this phenomenon. During REM sleep, NPT frequency increases; improved sleep quality from glycine supplementation could theoretically increase NPT events in men on daily tadalafil.

The Sleep Trial Evidence

The crossover RCT published in Sleep and Biological Rhythms (N=11) that studied 3 g glycine before bed found that time to sleep onset decreased by a mean of 9 minutes and slow-wave sleep percentage increased compared with placebo [2]. No cardiovascular monitoring was included in that trial. A follow-up study in the Journal of Pharmacological Sciences confirmed glycine-induced core body temperature reduction as the mechanism, via cutaneous vasodilation in extremities [8].

Clinical Implication

Men using tadalafil 5 mg daily who also take 3 g glycine before bed are combining a mild vasodilator (tadalafil, steady-state) with a mild peripheral vasodilator and sleep promoter (glycine). For the average healthy man, this combination is unlikely to cause meaningful hypotension during sleep. For men with heart failure, severe autonomic dysfunction, or recent cardiovascular events, discuss the combination with a prescribing physician before starting glycine.

Glycine's Metabolic Effects in Men With ED

Erectile dysfunction and metabolic syndrome share common vascular pathology. Low plasma glycine is an independent biomarker in men with obesity, insulin resistance, and cardiovascular risk [4]. Some clinicians use glycine supplementation as part of a broader metabolic optimization strategy alongside tadalafil for ED.

Plasma Glycine as a Biomarker

The PLOS ONE study cited above measured fasting plasma glycine in 200 men with type 2 diabetes and 200 normoglycemic controls. Diabetic men had 23% lower plasma glycine concentrations (mean 196 vs. 254 micromol/L, P<0.001) [4]. Lower plasma glycine correlated with higher fasting insulin, higher triglycerides, and greater waist circumference.

Potential Benefit for Vasculogenic ED

Vasculogenic ED, the most common organic form, involves impaired endothelial function and reduced NO bioavailability [9]. Glycine's role in glutathione synthesis may support endothelial antioxidant defense [6]. A 2021 review in Amino Acids described glycine as "conditionally essential" in metabolic disease states, noting its anti-inflammatory and pro-endothelial properties in preclinical models [10]. No RCT has tested glycine as an adjunct to PDE5 inhibitors for ED specifically, but the mechanistic overlap is plausible.

Dosing, Timing, and Practical Guidance

For men currently using both compounds, the practical recommendations are straightforward. No mandatory dose-separation window is required, because there is no pharmacokinetic conflict.

Recommended Glycine Doses by Use Case

• Sleep support: 3 g glycine taken 30 to 60 minutes before bed [2]
• Collagen synthesis: 5 to 10 g glycine daily, often with vitamin C to support hydroxylation [3]
• Metabolic support: 3 to 5 g daily with meals; effect on glycemic markers seen at 3 months in the PLOS ONE trial [4]

Tadalafil Dosing Context

• On-demand ED: 10 mg (titrate to 20 mg if needed), taken 30 to 60 minutes before activity [1]
• Daily ED or BPH: 5 mg at the same time each day; steady-state reached in 5 days [1]
• No dose adjustment of tadalafil is indicated when adding glycine at the doses above

Blood Pressure Monitoring

Check resting blood pressure before starting the combination if the patient uses any of the following: alpha-blockers, antihypertensive medications, or has a history of orthostatic hypotension. The American Heart Association 2023 guidelines on sexual activity and cardiovascular disease recommend that men with uncontrolled hypertension (systolic >170 mmHg) avoid PDE5 inhibitors until blood pressure is controlled [11]. Adding glycine does not change that threshold but reinforces the value of routine blood pressure measurement.

What the FDA and Major Guidelines Say

The FDA-approved prescribing information for tadalafil (Cialis) does not list glycine or any amino acid supplement in its drug interaction section [1]. The Natural Medicines database classifies the glycine-tadalafil interaction as "insufficient evidence to rate," which does not mean harm is expected but rather that no formal interaction studies exist [12].

Guideline Positions on PDE5 Inhibitor Supplements

The American Urological Association (AUA) 2018 guideline on ED management notes that supplements claiming to enhance sexual function should be evaluated for interactions with PDE5 inhibitors, particularly products containing yohimbine, L-arginine, or herbal nitrate donors [9]. Glycine does not appear on this caution list. L-arginine is specifically flagged as a potential additive hypotensive agent when combined with PDE5 inhibitors; glycine's hemodynamic profile is considerably milder [9].

Endocrine Society Guidance

The Endocrine Society's 2010 clinical practice guideline on male hypogonadism (updated 2018) acknowledges that men with testosterone deficiency commonly have comorbid ED and metabolic dysfunction [13]. Glycine's metabolic effects may complement testosterone replacement and tadalafil use in this population, though no guideline specifically addresses this triple combination.

Populations Requiring Extra Caution

Most healthy men can combine glycine and tadalafil without clinical concern. Three groups warrant closer monitoring.

Men on Nitrates

Tadalafil is absolutely contraindicated with organic nitrates (nitroglycerin, isosorbide mononitrate) due to severe synergistic hypotension [1]. This contraindication is unrelated to glycine but represents the most important safety rule for any man on tadalafil. Glycine does not contain nitrate groups and does not activate guanylate cyclase directly; it is not subject to this contraindication.

Men With Severe Renal Impairment

Tadalafil clearance is prolonged in severe renal impairment (creatinine clearance <30 mL/min); the maximum recommended dose is 5 mg in this population [1]. Glycine renal handling is also altered in severe renal disease, where glycine may accumulate [5]. Both compounds should be used with nephrologist input in dialysis patients.

Older Adults Over 65

Men over 65 have slower hepatic CYP3A4 activity and are more prone to orthostatic hypotension. The combined mild antihypertensive effect of glycine and tadalafil may produce more symptomatic blood pressure drops in this group. Starting tadalafil at 5 mg daily and glycine at 3 g before bed, with sitting-to-standing blood pressure checks for the first week, is a practical approach.

Summary of the Evidence

The table below organizes the key findings.

| Parameter | Glycine | Tadalafil | Combined Effect | |---|---|---|---| | CYP3A4 interaction | None | Substrate | No change in tadalafil levels | | Blood pressure | Small reduction (2 to 4 mmHg) | Moderate reduction (8 to 10 mmHg) | Additive, clinically small | | Sleep quality | Improves (3 g RCT) | No direct effect | Potentially additive benefit | | Nitric oxide pathway | Indirect support via glutathione | Dependent on NO-cGMP | Theoretical mild potentiation | | Contraindication | None identified | Nitrates (absolute) | Glycine does not trigger nitrate contraindication | | Recommended monitoring | Blood pressure if on vasodilators | Blood pressure, vision | Blood pressure |