Can I Take Glycine with Viagra (Sildenafil)?

What Glycine Is and What Doses Have Been Studied

Glycine is a non-essential amino acid sold over the counter as a dietary supplement [7][8]. Drugs@FDA lists it in approved prescription irrigation and injectable products, for example a 1.5% irrigation solution approved in 1978 [8].

Human trials have used it at doses that differ by more than an order of magnitude. Healthy volunteers whose sleep was cut by about 25% for three nights took 3 g before bed [1]. A 114-person trial in healthy older adults used glycine plus N-acetylcysteine at 2.4, 4.8 or 7.2 g a day total for two weeks [5]. Schizophrenia trials used 30 to 60 g a day [2][3]. These are study doses, not recommended doses.

What the Viagra Label Lists as Interactions and Contraindications

The interaction and contraindication sections of the Viagra label cover a short, specific list:

• Nitrates and organic nitrites in any form are contraindicated, and the label says it is unknown when nitrates can safely be given after a dose of Viagra [4].
• Riociguat, a guanylate cyclase stimulator, is contraindicated [4].
• Alpha-blockers: the label says the blood pressure effects add, that patients should be stable on the alpha-blocker before starting Viagra, and that Viagra should be started at 25 mg. With doxazosin, Viagra 25 mg produced a placebo-subtracted maximum standing systolic fall of 6.0 mmHg, and one of four men given 100 mg had severe postural hypotension [4].
• Amlodipine: an additional mean supine fall of 8/7 mmHg in hypertensive patients given Viagra 100 mg [4].
• Ritonavir raised sildenafil exposure 11-fold, and the label caps the dose at 25 mg in 48 hours. Erythromycin raised peak concentration 160% and AUC 182%; saquinavir raised them 140% and 210% [4].
• Alcohol at a mean peak blood alcohol of 0.08% did not add to sildenafil's blood pressure effect [4].

Glycine is not on that list [4].

The Limited Clozapine Interaction Signal

Glycine's interaction record is thin and comes from psychiatry. In a 19-patient double-blind trial, glycine 30 g a day added to clozapine for 12 weeks did not reduce positive or negative symptoms, while the patients on clozapine alone had a 35% reduction in positive symptoms; the authors suggested glycine may interfere with clozapine's effect [2]. A second double-blind trial of 30 adults used 60 g a day for 8 weeks and found no significant change in symptoms or cognitive functioning, and no patient had clinically significant worsening [3]. Both trials used doses far above the 3 g studied for sleep [1].

Blood Pressure: The Overlap Worth Knowing

Sildenafil relaxes smooth muscle through the nitric oxide and cGMP pathway, and it has systemic vasodilatory effects [4]. In healthy volunteers, single 100 mg doses lowered supine blood pressure by a mean maximum of about 8.4/5.5 mmHg, and the effect was not dose-related between 25 and 100 mg [4]. The label has no controlled data in patients with resting blood pressure below 90/50 mmHg or above 170/110 mmHg [4].

Glycine's blood pressure data come from a different group. In a trial of 60 people with metabolic syndrome, 15 g a day for 3 months produced a significant fall in systolic pressure among the men given glycine (P=0.043), along with a 25% fall in a lipid-peroxidation marker [6]. The report gives the significance, not the size of the fall.

Sildenafil Warnings That Apply Regardless of Glycine

The Viagra label says patients should seek immediate medical help for an erection lasting more than 4 hours, and that priapism is a painful erection lasting more than 6 hours that can cause permanent loss of potency if untreated [4]. It says to stop the drug and seek medical care for sudden loss of vision in one or both eyes, which could be non-arteritic anterior ischemic optic neuropathy, and to seek prompt attention for sudden decrease or loss of hearing [4]. It also warns that blood pressure can drop when sildenafil is combined with alpha-blockers or other antihypertensives [4]. The Viagra label says not to combine Viagra with Revatio or other PDE5 inhibitors because the combinations have not been studied and may further lower blood pressure [4][9].

The Practical Answer

Glycine is not among the interactions on the Viagra label [4]. Viagra has documented blood pressure effects and label cautions with alpha-blockers and other antihypertensives [4]. Glycine also lowered systolic pressure in men with metabolic syndrome in one trial at 15 g a day [6]. Whether the two effects add up when both are used has not been measured.