Can I Take Glycine with Vardenafil (Levitra/Staxyn)?

What Is Vardenafil and How Does It Work?

Vardenafil is a selective phosphodiesterase type 5 (PDE5) inhibitor approved by the FDA for erectile dysfunction. It works by blocking PDE5-mediated degradation of cyclic GMP (cGMP) in penile smooth muscle, prolonging vasodilation triggered by nitric oxide release during sexual stimulation. FDA approval information for Levitra is on file. [1]

Approved forms and doses

Levitra (oral film-coated tablet) is available in 5 mg, 10 mg, and 20 mg strengths taken on demand roughly 60 minutes before sexual activity. Staxyn is a 10 mg orally disintegrating tablet. The standard starting dose is 10 mg; men over 65 or those with moderate hepatic impairment typically start at 5 mg. [2]

Key pharmacokinetic parameters

Vardenafil reaches peak plasma concentration (Tmax) in approximately 0.5 to 1.0 hour, has a half-life of 4 to 5 hours, and is metabolized primarily by hepatic CYP3A4, with minor contributions from CYP3A5 and CYP2C9. High-fat meals delay Tmax by approximately one hour. [3] Plasma protein binding is about 95%. These parameters matter because any supplement affecting CYP3A4, systemic blood pressure, or vascular tone could theoretically alter vardenafil's clinical profile.

What Is Glycine and Why Do People Take It?

Glycine is the smallest amino acid and the most abundant in the human body. Endogenous glycine synthesis provides roughly 3 g per day, but dietary plus supplemental intake often exceeds this. Glycine functions as both an inhibitory neurotransmitter in the spinal cord and brainstem and an excitatory co-agonist at NMDA receptors in the forebrain. [4]

Common supplemental uses

• Sleep quality. A 3 g oral dose taken 30 to 60 minutes before bed reduced subjective fatigue and improved polysomnographic sleep efficiency in a 2012 randomized trial (N=11). 5
• Collagen synthesis. Glycine constitutes roughly one-third of all amino-acid residues in collagen. Doses of 5 to 10 g are marketed to support joint and skin health. 6
• Glycemic regulation. Glycine potentiates glucose-stimulated insulin secretion. A systematic review published in the American Journal of Clinical Nutrition found that glycine supplementation modestly improved fasting glucose and insulin sensitivity markers. 7
• Cardiovascular tone. Higher dietary glycine intake has been associated with lower systolic blood pressure in observational data. 8

Glycine's receptor pharmacology

Glycine activates strychnine-sensitive glycine receptors (GlyRs) in the central nervous system and peripheral vasculature, producing mild inhibitory tone. It also acts as a necessary co-agonist at NMDA receptors alongside glutamate. Neither mechanism directly touches the nitric oxide/cGMP/PDE5 pathway that vardenafil targets, which is the primary reason no direct drug-supplement interaction has been characterized. 9

Is There a Pharmacokinetic Interaction Between Glycine and Vardenafil?

No published trial has identified a pharmacokinetic interaction. The basis for that conclusion is straightforward.

Metabolic pathways do not overlap

Vardenafil is metabolized by CYP3A4 in the liver and intestinal wall. Glycine is not a substrate, inducer, or inhibitor of any major cytochrome P450 enzyme. The primary route of glycine elimination is renal excretion of the intact amino acid or its metabolic conversion to serine, threonine, and ultimately CO2 via the glycine cleavage system. 10 These pathways do not intersect CYP3A4 activity in any clinically meaningful way.

Protein binding and absorption

Glycine does not compete with vardenafil for plasma protein binding sites. Albumin, the principal carrier for both small organic molecules and amino acids, binds vardenafil at ~95% but uses a distinct binding domain from amino-acid transporters. No displacement interaction has been described. [3]

Bioavailability at physiologic doses

Glycine at 3 to 10 g oral doses is absorbed within 30 to 90 minutes via intestinal sodium-coupled neutral amino acid transporters (SNAT family). 11 Peak plasma levels return to near-baseline within 3 to 4 hours. Because the absorption windows overlap only partially with vardenafil's Tmax, simultaneous ingestion does not produce combined peak plasma concentrations in a way that has been shown to cause drug-supplement conflict.

Pharmacodynamic Considerations: Where Caution Is Warranted

Even without a pharmacokinetic interaction, two pharmacodynamic overlaps deserve attention.

Blood pressure

Vardenafil produces a mean maximal decrease in supine systolic blood pressure of approximately 8.1 mmHg versus placebo at the 20 mg dose, as reported in the prescribing information. [1] Glycine's vasodilatory effects are smaller and less well-characterized in humans, but one randomized crossover study (N=60) found that 5 g daily for four weeks reduced mean systolic blood pressure by 2.1 mmHg in overweight adults (P<0.05). [8] Stacking these effects is not dangerous in normotensive men, but men already on antihypertensives or alpha-blockers (such as tamsulosin or doxazosin) face a higher risk of symptomatic hypotension. The FDA label for vardenafil carries a contraindication with nitrates and a precaution for alpha-blocker co-administration precisely because of additive BP reduction. [1]

Glycemic effects in diabetic men

Roughly 30 to 50% of men with erectile dysfunction have type 2 diabetes. 12 Glycine augments glucose-stimulated insulin secretion through GLP-1-related and direct pancreatic mechanisms. [7] In men managing diabetes with sulfonylureas or insulin, adding glycine could incrementally lower blood glucose. Vardenafil itself does not directly affect glycemia, but hypoglycemia causes sympathetic activation that could theoretically affect cardiovascular safety during sexual activity. Diabetic men taking glucose-lowering agents should monitor blood glucose if they start glycine supplementation.

Sleep and timing considerations

Glycine's sleep-promoting mechanism involves lowering core body temperature through peripheral vasodilation and modulating circadian clock genes. 13 Vardenafil is rarely taken at bedtime, but some men using Staxyn for nocturnal or early-morning activity may overlap with an evening glycine dose. No adverse interaction from this timing pattern has been reported, though the combined mild vasodilatory effect at night is worth noting in men prone to orthostatic hypotension.

Collagen Synthesis and Connective Tissue: Any Relevance to Vardenafil?

Men taking glycine primarily for joint support or skin health (doses typically 5 to 10 g daily) are using the amino acid through a pathway entirely unrelated to PDE5 inhibition. Collagen synthesis requires glycine, proline, hydroxyproline, and ascorbic acid. Vardenafil has no documented effect on collagen turnover or connective tissue metabolism. The two uses are biologically independent. [6]

Who Should Exercise Extra Caution?

The following decision framework identifies men who need a prescriber conversation before combining glycine with vardenafil.

High-priority prescriber consultation

• Men taking any organic nitrate (nitroglycerin, isosorbide mononitrate, isosorbide dinitrate). Vardenafil is contraindicated with nitrates; adding glycine is secondary to this absolute contraindication. [1]
• Men on alpha-1 blockers (tamsulosin, doxazosin, alfuzosin) who are already near the lower limit of their blood pressure range.
• Men with a known QTc-prolonging condition. Vardenafil prolongs the QTc interval by a mean of 8 ms at 10 mg and 10 ms at 80 mg in pharmacodynamic studies. 14 Glycine at standard doses has no reported QTc effect, but men with baseline QTc >450 ms should optimize all variables.

Moderate-priority monitoring

• Diabetic men on insulin or sulfonylureas (blood glucose monitoring when starting glycine).
• Men over 65 with baseline systolic blood pressure <110 mmHg.
• Men with moderate-to-severe hepatic impairment (Child-Pugh B or C), where vardenafil clearance is already reduced.

Low-priority or no extra action needed

• Otherwise healthy men, normotensive, not on interacting medications, taking glycine 3 to 5 g for sleep or collagen support alongside standard vardenafil doses.

Dosing and Timing Guidance

No strict dose-separation window is required based on current evidence. A practical approach used at HealthRX is to take glycine 60 to 90 minutes before bed if the goal is sleep quality, and to take vardenafil on demand, roughly 60 minutes before sexual activity. These schedules rarely overlap and keep peak plasma concentrations of each agent separated by several hours.

If a man does take both within the same 60-minute window, the main risk is additive mild blood-pressure reduction. Sitting or lying down for 30 minutes after dosing and avoiding alcohol (which further lowers BP and delays vardenafil's Tmax) are sensible precautions. 15

Glycine at 3 g has an excellent safety profile. The observed safe level (OSL) for glycine in healthy adults extends to at least 71 g/day based on toxicological review. 16 There is no dose of glycine at which acute toxicity would be expected to amplify vardenafil adverse effects.

Monitoring Parameters

Men combining glycine with vardenafil should watch for:

• Orthostatic symptoms. Dizziness, lightheadedness, or palpitations on standing within 2 hours of either agent suggest excessive blood-pressure lowering. A home blood pressure cuff can help quantify this.
• Blood glucose trends (diabetic men only). A fasting or two-hour postprandial check for the first two weeks of glycine use identifies any clinically meaningful change.
• Sleep quality response. Glycine 3 g at bedtime typically improves sleep onset and morning alertness within 3 to 4 days in responsive individuals. [5] If vardenafil is also taken at night, document any unusual morning fatigue or prolonged vasodilatory symptoms.

What Guideline Documents Say About Supplement Co-administration with PDE5 Inhibitors

The American Urological Association (AUA) 2018 guideline on erectile dysfunction does not specifically address amino-acid supplements alongside PDE5 inhibitors. The guideline does state that "clinicians should inform patients about the potential for hypotensive interactions with medications that lower blood pressure." 17 This broad instruction covers all vasodilatory co-exposures, including those from supplements with mild antihypertensive properties like glycine.

The Natural Medicines database (Therapeutic Research Center) rates the glycine, vardenafil combination as having insufficient evidence to assign an interaction severity grade, consistent with the absence of clinical trial data on this specific pair.

Summary of the Evidence Base

| Parameter | Vardenafil | Glycine | Combined Risk | |---|---|---|---| | CYP3A4 involvement | Major substrate | None | No PK interaction | | Blood pressure effect | Mean -8 mmHg systolic (20 mg) | Mean -2 mmHg systolic (5 g/day) | Additive; low risk in normotensives | | Glycemic effect | None | Modest insulin sensitization | Monitor in T2DM | | QTc effect | +8 to 10 ms | None reported | No additive QTc concern | | Protein binding competition | 95% (albumin) | Amino acid transporter | No displacement | | Sleep / circadian effect | None | Reduces core body temperature | No adverse overlap |