Can I Take Glycine with Viagra (Sildenafil)?

What Is Glycine and Why Do People Take It?

Glycine is the simplest amino acid, found in bone broth, gelatin, and collagen peptides, and sold as a standalone powder or capsule. Adults synthesize roughly 3 g per day endogenously, yet dietary intake often falls short of the estimated 10 g per day needed to support all physiological demands. [1]

Common Uses

People use supplemental glycine for three main reasons: sleep quality, collagen synthesis support, and glycemic regulation. A randomized crossover trial in 11 healthy volunteers showed that 3 g of glycine taken 1 hour before bedtime reduced core body temperature and shortened sleep onset, with subjects reporting less daytime sleepiness the following morning. [2] Separately, glycine contributes to collagen triple-helix stability because every third residue in collagen is a glycine; higher circulating glycine supports fibroblast synthesis rates. [3]

Glycemic Effects

Glycine stimulates glucagon-like peptide-1 (GLP-1) and insulin secretion in a glucose-dependent manner, which lowers post-meal blood glucose without triggering hypoglycemia in normoglycemic individuals. [4] This effect is modest at doses below 5 g and is not considered clinically significant in the absence of antidiabetic drugs.

Nitric Oxide Connection

Glycine serves as a precursor to glutathione and participates indirectly in nitric oxide (NO) regulation by supporting endothelial antioxidant capacity. [5] This is the pathway most relevant to sildenafil co-administration, discussed below.

How Sildenafil Works: The PDE5 Pathway

Sildenafil is a selective phosphodiesterase type-5 (PDE5) inhibitor. Sexual stimulation triggers endothelial NO release in penile tissue. NO activates soluble guanylate cyclase, raising cyclic guanosine monophosphate (cGMP). CGMP relaxes smooth muscle and increases blood flow, producing erection. Sildenafil blocks PDE5, the enzyme that degrades cGMP, prolonging its vasodilatory effect. [6]

Pharmacokinetic Profile

After a 100 mg oral dose, sildenafil reaches peak plasma concentration (Tmax) at roughly 60 minutes, with a bioavailability of approximately 40% due to first-pass metabolism. The half-life is 3 to 5 hours. CYP3A4 handles the majority of hepatic clearance; CYP2C9 contributes a minor fraction. [7] A high-fat meal delays Tmax by up to 60 minutes and reduces peak concentration by 29%, which is why the Viagra prescribing information recommends taking it on an empty or light-fat stomach. [8]

Systemic Blood Pressure Effects

Even at the approved 100 mg dose, sildenafil produces a mean maximum decrease in supine systolic blood pressure of approximately 8 to 10 mmHg compared with placebo in FDA registration studies. [8] This effect is amplified significantly when sildenafil is combined with nitrates (an absolute contraindication) or alpha-blockers (requires dose separation). [8]

Does Glycine Interact with Sildenafil? The Pharmacokinetic Evidence

No pharmacokinetic interaction between glycine and sildenafil has been published in peer-reviewed literature as of mid-2025. The reason is mechanistic: glycine is not metabolized by, nor does it inhibit, CYP3A4 or CYP2C9. [9]

CYP Enzyme Assessment

CYP3A4 inhibitors such as ketoconazole (200 mg) raise sildenafil area under the curve (AUC) by roughly 182%, requiring sildenafil dose reduction to 25 mg. [8] CYP3A4 inducers such as rifampicin reduce sildenafil AUC by approximately 74%. [8] Glycine exerts neither effect. In vitro data on amino acids confirm that glycine at physiological concentrations (50 to 500 µM plasma range) does not meaningfully alter CYP3A4 or CYP2C9 enzymatic activity. [9]

Protein Binding

Sildenafil is approximately 96% plasma-protein-bound. [7] Amino acids can theoretically compete for albumin binding sites, but glycine's affinity for albumin is orders of magnitude lower than sildenafil's; displacement at supplement doses is not considered clinically relevant. [10]

Renal Clearance

Glycine is freely filtered and largely reabsorbed in the proximal tubule. It does not alter renal tubular secretion pathways used by sildenafil metabolites. [1]

Pharmacodynamic Overlap: Where Caution Applies

Although the pharmacokinetic interaction is negligible, both compounds touch cardiovascular physiology, and the overlap deserves explicit discussion.

Nitric Oxide Signaling

Glycine at doses of 2 to 5 g may support endothelial NO bioavailability by replenishing glutathione, the key antioxidant that prevents NO from being scavenged by reactive oxygen species. [5] A 2016 study (N=60 patients with metabolic syndrome) found that glycine supplementation at 15 g/day for 3 months significantly improved plasma glutathione levels and reduced oxidative stress markers compared with placebo (P<0.01). [11] Higher endothelial NO availability could theoretically potentiate sildenafil's cGMP-dependent vasodilation. At the 3 to 5 g doses most men use, this effect is speculative and likely small.

Blood Pressure Additive Effect

Glycine at 5 g/day modestly reduced systolic blood pressure in a small randomized trial (N=35, mean reduction 3.1 mmHg vs. Placebo). [12] Layered on sildenafil's own 8 to 10 mmHg systolic reduction, total blood pressure lowering could reach 11 to 13 mmHg in sensitive individuals. This is not dangerous for most men, but men with baseline systolic blood pressure below 90 mmHg, those on antihypertensive therapy, or those taking alpha-blockers for lower urinary tract symptoms should discuss this combination with their prescriber.

Sleep Timing and Dose Separation

Most glycine users take 3 g at bedtime to improve sleep. Sildenafil for erectile dysfunction is taken 30 to 60 minutes before anticipated sexual activity. These windows rarely overlap. If a man takes sildenafil in the evening and then glycine later at sleep time, he should be aware that sildenafil's half-life of 3 to 5 hours means measurable plasma levels persist 4 to 6 hours post-dose. Blood pressure in the lowest quartile after sildenafil's peak effect (1 to 2 hours post-dose) will have largely recovered by typical bedtime dosing of glycine. The practical risk is very low.

Glycine and Collagen: Relevance to Men on Sildenafil

Several men who use sildenafil also take collagen peptides for joint or skin support. Collagen peptides are hydrolyzed into free amino acids, of which glycine is the dominant constituent (approximately 330 mg of glycine per gram of collagen peptide). [3]

Daily Glycine Load from Collagen Supplements

A man taking 10 g of collagen peptide powder receives roughly 3.3 g of glycine from that source alone, before adding any standalone glycine supplement. Total daily glycine from diet, collagen, and supplement can reach 15 to 20 g in some users. [1] No safety signal has been identified at these doses in healthy adults. The European Food Safety Authority classifies glycine as safe at intakes up to 470 mg/kg/day in adults. [13]

Special Populations and Risk Stratification

Not every man on sildenafil carries the same risk profile. The table below provides a practical framework for assessing glycine co-administration risk, intended to guide the prescriber-patient conversation rather than replace it.

| Patient Profile | Glycine Dose | Risk Assessment | Recommended Action | |---|---|---|---| | Healthy man, normal BP, no other meds | 3 to 5 g at bedtime | Very low | No dose adjustment needed; disclose to prescriber | | Hypertension on ACE inhibitor | 3 to 5 g | Low-moderate | Monitor BP; avoid glycine within 2 hours of sildenafil | | Alpha-blocker for BPH | 3 to 5 g | Moderate | Discuss with prescriber; consider timing separation | | Baseline systolic BP <90 mmHg | Any dose | Moderate | Use with caution; start glycine at 1 to 2 g | | Pulmonary arterial hypertension on Revatio | >5 g | Unknown | Consult prescribing cardiologist/pulmonologist |

What the FDA Label Says About Sildenafil Drug Interactions

The FDA-approved prescribing information for Viagra identifies the following interaction categories [8]:

• Absolute contraindication: nitrates in any form (risk of severe hypotension)
• Avoid or use with caution: riociguat, strong CYP3A4 inhibitors (ketoconazole, itraconazole, ritonavir), alpha-blockers (timing separation required)
• Dose adjustment: CYP3A4 inducers (rifampin reduces AUC by approximately 74%)
• Not listed: amino acids, including glycine

The absence of glycine from the interaction table reflects the absence of any documented interaction, not an absence of scrutiny. The FDA label was last updated in 2014; no post-marketing safety reports specifically linking glycine to sildenafil adverse events appear in the FDA Adverse Event Reporting System (FAERS) database search as of this writing. [14]

Glycine and Glycemic Control: Relevance in Men with Type 2 Diabetes

Erectile dysfunction is more prevalent in men with type 2 diabetes: the Massachusetts Male Aging Study found ED prevalence of 52% in men aged 40 to 70, with diabetes as an independent predictor. [15] Many of these men use sildenafil. Glycine's GLP-1-stimulating effect could, in theory, interact with antidiabetic medications, but this is a drug-supplement-drug interaction involving the antidiabetic agent, not sildenafil itself. [4] Sildenafil does not have meaningful glycemic effects at standard doses.

Practical Dosing Guidance for Co-Administration

Taking glycine and sildenafil together does not require dose adjustment based on current evidence. For men who choose to use both:

• Take sildenafil 50 to 100 mg approximately 60 minutes before sexual activity on a light-fat meal. [8]
• Take glycine 3 g at bedtime for sleep support, typically 4 to 6 hours after sildenafil in most evening-use scenarios.
• If taking both within 2 hours of each other, sit up slowly after lying down to minimize orthostatic hypotension risk, particularly in men over age 65.
• Measure resting blood pressure before starting glycine if you are on antihypertensive therapy or if your most recent systolic reading was below 100 mmHg.
• Disclose glycine supplementation at every sildenafil prescription refill visit so your provider can update your medication reconciliation list.

Monitoring Parameters

No laboratory monitoring is specifically required for glycine-sildenafil co-administration. Clinically, the relevant monitoring is blood pressure, especially in the first few uses of the combined regimen. A home blood pressure cuff reading of systolic below 85 mmHg or diastolic below 50 mmHg after taking both agents should prompt the patient to sit or lie down and contact their prescriber. [8]

When to Stop and Call Your Doctor

Stop glycine and contact your prescriber if you experience: lightheadedness or fainting after taking sildenafil at any time you are also using glycine, a sustained drop in blood pressure readings compared with your baseline, or any new cardiovascular symptoms such as chest pain or palpitations. These symptoms are more likely caused by sildenafil alone or an underlying cardiovascular condition than by glycine, but the combination warrants clinical evaluation.

Evidence Gaps and Ongoing Research

No randomized controlled trial has directly tested the glycine-sildenafil combination. The gap in evidence is not surprising. Amino acid-drug pharmacokinetic studies are rarely funded because amino acids are not patentable. The interaction most worth studying is the pharmacodynamic question of whether glycine's endothelial NO support meaningfully potentiates sildenafil's vasodilation at doses above 10 g/day. A crossover study measuring penile blood flow (using duplex Doppler ultrasonography) in men given sildenafil 50 mg with and without glycine preloading at 10 g would cost-effectively resolve this question. The HealthRX medical team rates the current evidence certainty for "no clinically meaningful interaction at 3 to 5 g glycine" as moderate, based on mechanistic plausibility and absence of safety signals, rather than high-quality head-to-head trial data.

The American Urological Association 2018 guideline on erectile dysfunction does not mention supplement-drug interactions beyond nitrates and alpha-blockers. [16] The Endocrine Society similarly does not address amino acid co-administration in its testosterone and sexual function guidance. [17] These omissions reflect the general evidence gap, not clinical endorsement.