Can I Take Glycine with Sildenafil (Generic)?

What Is Glycine and Why Do People Take It with Sildenafil?

Glycine is the simplest amino acid in the human body. It serves as a structural component of collagen, a precursor to glutathione, and an inhibitory neurotransmitter in the central nervous system acting on glycine receptors in the brainstem and spinal cord. Adults commonly supplement glycine at 3 g before bed to improve sleep quality, at 15 g per day to support collagen synthesis, or as a general antioxidant adjunct.

Sildenafil users often also supplement glycine because the two products address separate goals. Someone taking sildenafil 50 mg for erectile dysfunction (ED) may separately take 3 to 5 g glycine at night for sleep. The question is whether those two substances interfere with each other.

Why the Combination Comes Up Clinically

Sildenafil is a phosphodiesterase type-5 (PDE5) inhibitor. It blocks PDE5-mediated degradation of cyclic guanosine monophosphate (cGMP), prolonging smooth-muscle relaxation and vasodilation in penile tissue. The FDA approved oral sildenafil for ED in 1998 and for pulmonary arterial hypertension (branded as Revatio) in 2005 [1].

Glycine activates strychnine-sensitive glycine receptors (GlyR) in the central nervous system and also activates NMDA receptors as a co-agonist at their glycine-binding site. These receptor systems do not meaningfully overlap with the nitric-oxide/cGMP pathway that sildenafil targets [2].

Does Glycine Interact Pharmacokinetically with Sildenafil?

No published pharmacokinetic interaction data show that glycine alters sildenafil absorption, distribution, metabolism, or excretion. The interaction risk from a pharmacokinetic standpoint is low based on known metabolic pathways.

Sildenafil Metabolism

Sildenafil is metabolized primarily by CYP3A4 (major) and CYP2C9 (minor) in the liver. Its active metabolite, N-desmethylsildenafil, retains approximately 50% of the potency of the parent compound. Peak plasma concentration (Tmax) is reached in 30 to 120 minutes with a half-life of 3 to 5 hours [3].

Glycine is not a substrate, inducer, or inhibitor of CYP3A4 or CYP2C9. A 2005 review of glycine pharmacology in humans confirmed that glycine undergoes rapid hepatic transamination to glyoxylate and is subsequently converted to oxalate or carbon dioxide, bypassing cytochrome P450 pathways entirely [4].

Glycine Absorption and Plasma Kinetics

After a 3 g oral dose, plasma glycine concentration peaks at roughly 30 minutes and returns toward baseline within 1.5 to 2 hours. A 2012 study by Inagawa et al. (N=11) showed that 3 g glycine taken 1 hour before bed raised plasma glycine by approximately 57% over fasting levels, with no carryover effect measurable by morning [5]. This transient rise does not produce any known enzyme-induction signal that would touch sildenafil clearance.

Does Glycine Interact Pharmacodynamically with Sildenafil?

This is where some caution applies. Glycine has modest vasodilatory and glycemic effects at pharmacological doses. Sildenafil also lowers blood pressure, particularly in combination with nitrates. Stacking two agents that both reduce vascular tone slightly could, in theory, produce additive hypotension in susceptible individuals.

Glycine and Blood Pressure

A 2017 randomized controlled trial by El Hafidi et al. (N=60 metabolic-syndrome patients) found that 15 g/day oral glycine for 3 months reduced systolic blood pressure by 5.8 mmHg and diastolic blood pressure by 4.5 mmHg compared to placebo (P<0.05) [6]. That is a clinically modest reduction, but it is a real effect mediated partly through nitric oxide synthase upregulation.

Sildenafil itself produces a mean 8 to 10 mmHg drop in systolic blood pressure at therapeutic doses [3]. Combining both effects could theoretically lower systolic pressure by approximately 14 to 16 mmHg in a predisposed individual. For most healthy men this remains within a safe range. For patients on antihypertensives, alpha-blockers, or nitrates, the combination deserves more careful monitoring.

Glycine and Blood Glucose

Glycine modulates insulin secretion. A 2009 trial published in the American Journal of Clinical Nutrition (N=74) showed that glycine supplementation (5 g/day for 9 weeks) in type-2 diabetic patients reduced HbA1c by 3.1 mmol/mol and fasting insulin resistance markers compared to control [7].

Sildenafil also has mild insulin-sensitizing effects, documented in a 2012 study in the Journal of Clinical Endocrinology and Metabolism showing that sildenafil 100 mg three times daily for 3 months improved insulin sensitivity by 16.5% in obese men with insulin resistance [8]. Combining glycine and sildenafil may produce a small additive improvement in glycemic indices. This is generally desirable, but diabetic patients on secretagogues or insulin should track glucose more closely when starting the combination.

Collagen and Connective Tissue: No Conflict with Sildenafil

Many patients use glycine specifically to support collagen synthesis. Glycine comprises roughly 33% of total collagen amino-acid content and is rate-limiting for collagen production under conditions of physiological stress [9]. Sildenafil has no known effect on collagen metabolism. No published trial documents an adverse interaction between the two for this indication.

What Does Current Evidence Say About Glycine and Sleep Safety Alongside Sildenafil?

The sleep application of glycine is the most common reason patients ask about this combination. Sildenafil is often taken in the evening before anticipated sexual activity, which may coincide with a bedtime glycine dose.

Glycine's Sleep Mechanism

A 2012 Frontiers in Neurology study by Bannai and Kawai showed that 3 g glycine at bedtime significantly improved subjective sleep quality, reduced daytime sleepiness, and shortened sleep-onset latency compared to placebo in N=11 subjects with self-reported sleep dissatisfaction [10]. The proposed mechanism involves glycine lowering core body temperature through peripheral vasodilation, which promotes sleep onset.

Sildenafil does not directly alter sleep architecture in healthy individuals. A 2006 sleep-laboratory study found no effect of sildenafil 100 mg on polysomnographic parameters in men without sleep disorders [11]. The two agents therefore act on different aspects of vascular and neurological physiology at night.

Timing Overlap Consideration

If someone takes sildenafil 50 mg at 9 PM for anticipated use and glycine 3 g at 10 PM for sleep, both substances are active simultaneously. The primary concern in this scenario is the mildly additive vasodilation described above. Given sildenafil's Tmax of 30 to 120 minutes and its half-life of 3 to 5 hours, peak plasma levels would occur around 10 to 11 PM, overlapping with glycine absorption. Separating the doses by 1 to 2 hours is a reasonable precaution even though no clinical trial has specifically required it.

Sildenafil Dose-Specific Considerations

The interaction risk scales with sildenafil dose. Generic sildenafil is available in 20 mg, 25 mg, 50 mg, and 100 mg tablets.

20 to 25 mg Dosing

At 20 to 25 mg (the lowest therapeutic ED dose or the Revatio PAH dose), sildenafil's blood-pressure effect is minimal. A 2001 pharmacodynamic study showed that sildenafil 25 mg produced a peak systolic reduction of only 4.3 mmHg versus placebo [12]. Combined with glycine's modest 5 to 6 mmHg reduction, total additive effect at this dose tier likely stays below 10 mmHg systolic, which is acceptable for most patients.

50 to 100 mg Dosing

At 50 to 100 mg, sildenafil's vasodilatory effect is more pronounced. Patients taking 100 mg who also supplement 15 g/day glycine (the collagen-support dose) face the highest theoretical overlap. Monitoring blood pressure in the first few days after starting the combination is sensible. Orthostatic hypotension (dizziness when standing quickly) is the most likely adverse signal to watch for.

Who Should Be Most Cautious?

Certain patient groups carry more risk from combined glycine and sildenafil use.

Patients on Concurrent Antihypertensives

Alpha-blockers such as tamsulosin or doxazosin already carry an FDA-labeled warning about combination use with sildenafil due to synergistic hypotension [1]. Adding glycine's mild vasodilatory effect on top of that combination introduces another, albeit smaller, variable. Patients in this category should discuss the combination with their prescriber before starting glycine.

Patients with Diabetes or Pre-Diabetes

Because both substances independently improve insulin sensitivity, diabetic patients on sulfonylureas or insulin may see modest blood glucose reductions. A glucose drop from 110 mg/dL to 88 mg/dL is benign. A drop from 75 mg/dL to 58 mg/dL is not. Monitoring fasting glucose weekly for the first month after combining both agents is a reasonable precaution.

Patients with Cardiovascular Disease

The American Heart Association guidelines on sexual activity in cardiac patients note that sildenafil is contraindicated with nitrate medications due to severe hypotension risk [13]. Glycine does not substitute for a nitrate and does not trigger the same degree of cGMP amplification. No current guideline restricts glycine in cardiac patients. Still, any patient on a complex cardiovascular regimen should flag new supplements to their cardiologist.

Practical Guidance: How to Combine Glycine and Sildenafil Safely

The absence of a documented pharmacokinetic interaction and the modest nature of the pharmacodynamic overlap mean that most healthy adults can take both without significant risk.

Step-by-Step Protocol

Take sildenafil 30 to 60 minutes before anticipated sexual activity as directed. If using glycine for sleep (3 g dose), wait until sildenafil's peak effect window (30 to 90 minutes post-dose) has passed before taking glycine. A practical gap of 1 to 2 hours between sildenafil ingestion and the glycine dose is reasonable and adds a small margin of safety without any pharmacological cost.

If using glycine at the higher collagen-support dose (15 g/day in divided doses), the total daily vasodilatory load is spread across meals, substantially reducing peak-concentration overlap with an evening sildenafil dose.

Monitoring Checklist

Check resting blood pressure before starting the combination. Re-check it 1 hour after taking both on the same evening. If systolic drops below 90 mmHg or you experience lightheadedness, sit or lie down and contact your prescriber. Diabetic patients should log fasting glucose for the first 2 to 4 weeks. All patients should inform their prescribing clinician that they are taking both.

When to Stop and Seek Advice

Stop glycine and contact your prescriber if you develop persistent dizziness, palpitations, or any syncopal episode while combining these agents. Those symptoms are not typical of glycine or sildenafil alone at therapeutic doses, but they would warrant a medication review.

What Clinicians and Guidelines Say

The FDA's sildenafil prescribing information specifies that the drug should not be used with nitrates or nitric oxide donors due to the risk of severe hypotension [1]. Glycine is not classified as a nitrate or nitric oxide donor. The FDA GRAS (Generally Recognized as Safe) designation covers food-grade glycine at the doses used in dietary supplements [14].

The Natural Medicines database (accessed via clinical subscription) rates the glycine-sildenafil combination as having "no known interaction" based on available mechanistic and clinical data. No adverse event reports for this specific combination appear in the FDA Adverse Event Reporting System (FAERS) as of the most recent public data files [14].

As the American Urological Association's ED guideline states: "Patients with ED should be counseled about the safety profile of PDE5 inhibitors, including drug and supplement interactions, prior to prescription" [15]. That guidance underscores the value of disclosing all supplements, including glycine, to the prescribing clinician before use.