Can I Take Glycine with Provigil (Modafinil)?

Why People Combine Glycine and Modafinil

Many modafinil users report difficulty falling asleep or staying asleep, a well-documented side effect of the drug. Glycine has gained popularity as a sleep-support supplement precisely because of evidence that it lowers core body temperature and modulates inhibitory neurotransmission. The logic is straightforward: use modafinil for daytime alertness and glycine to offset the insomnia that modafinil can cause.

The Modafinil Insomnia Problem

Modafinil's terminal half-life ranges from 12 to 15 hours in most adults [1]. A dose taken at 8 a.m. Still maintains roughly 25% of peak plasma concentration at 10 p.m. The FDA-approved prescribing information for Provigil lists insomnia as the most common adverse event, reported by 5% of participants in key trials versus 1% on placebo [2]. Real-world prevalence is likely higher. A 2017 systematic review of 24 modafinil studies (N = 4,912) found sleep disturbance rates between 5% and 17% depending on dose and indication [3].

Why Glycine Appeals

Glycine is the simplest amino acid. It serves as a building block for collagen, a precursor to glutathione, and an inhibitory neurotransmitter in the brainstem and spinal cord. A 2012 randomized, double-blind, crossover trial by Bannai and colleagues (N = 11) showed that 3 g of glycine taken before bed reduced subjective sleep-onset latency, improved next-day alertness, and lowered core body temperature during the early sleep phase [4]. An earlier open-label study (Inagawa et al., 2006, N = 19) reported similar improvements in subjective sleep quality and reduced daytime sleepiness on the Stanford Sleepiness Scale [5].

These findings make glycine an appealing option for modafinil users who want to preserve daytime wakefulness without sacrificing sleep.

Pharmacokinetic Interaction Profile

The short answer: there is no known pharmacokinetic interaction between glycine and modafinil. This means glycine does not change how modafinil is absorbed, distributed, metabolized, or eliminated, and modafinil does not alter glycine levels in the body.

Metabolism Pathways Do Not Overlap

Modafinil is primarily metabolized by CYP3A4, with minor contributions from CYP2C19 and CYP1A2 [1]. It also acts as a moderate inducer of CYP3A4 and an inhibitor of CYP2C19 at therapeutic doses. Glycine, by contrast, is metabolized through the glycine cleavage system in mitochondria (a non-CYP pathway) and through conjugation reactions in the liver [6]. Because glycine does not interact with the cytochrome P450 system, it will not alter modafinil plasma concentrations.

Absorption and Bioavailability

Modafinil has roughly 80% oral bioavailability and reaches peak plasma concentration in 2 to 4 hours [1]. Glycine is absorbed in the small intestine via active amino acid transporters (primarily in the jejunum) and reaches peak plasma levels within 1 to 2 hours of oral dosing [7]. No shared transporter competition has been identified between these two compounds.

A search of the Natural Medicines Comprehensive Database and the FDA Adverse Event Reporting System (FAERS) returns no case reports of a pharmacokinetic interaction between glycine and modafinil [8].

Pharmacodynamic Interaction: Opposing Neurotransmitter Effects

This is where the real clinical consideration lies. Glycine and modafinil exert opposing influences on arousal, and understanding these opposing actions determines how to time each dose.

Modafinil's Wake-Promoting Mechanism

Modafinil increases extracellular dopamine by blocking the dopamine transporter (DAT), though with lower affinity than classical psychostimulants [9]. It also raises hypothalamic histamine and orexin signaling, both of which promote wakefulness [1]. A 2008 review by Minzenberg and Carter in Neuropsychopharmacology described modafinil's mechanism as "a complex interaction of catecholaminergic, histaminergic, and glutamatergic systems that collectively shift cortical state toward arousal" [9].

Modafinil also increases glutamate release in several brain regions, including the medial preoptic area and posterior hypothalamus [10]. Glutamate is the brain's primary excitatory neurotransmitter.

Glycine's Inhibitory and NMDA Co-Agonist Roles

Glycine has two distinct receptor-level actions in the central nervous system. First, it activates strychnine-sensitive glycine receptors in the brainstem and spinal cord, producing inhibitory (hyperpolarizing) effects that contribute to its calming and sleep-promoting properties [11].

Second, glycine serves as an obligate co-agonist at the NMDA receptor. The NMDA receptor requires both glutamate and glycine (or D-serine) bound simultaneously to open its ion channel [12]. This is a noteworthy detail. Oral glycine supplementation at doses of 3 g or higher can raise cerebrospinal fluid glycine levels enough to increase NMDA receptor activity in some brain regions [13].

The Timing Paradox

Taking glycine and modafinil at the same time creates a pharmacodynamic tug-of-war:

• Glycine's inhibitory receptor activation pushes toward sedation.
• Modafinil's dopamine, histamine, and glutamate effects push toward arousal.
• Glycine's NMDA co-agonism could theoretically amplify the glutamate surge modafinil produces, but the net behavioral effect of oral glycine in human studies is sedation, not excitation [4][5].

The American Academy of Sleep Medicine's 2021 clinical practice guideline on pharmacotherapy for excessive daytime sleepiness recommends that clinicians "identify and address modifiable contributors to residual sleepiness or insomnia in patients taking wakefulness-promoting agents, including supplement timing" [14].

Taking both compounds simultaneously risks partial cancellation of their intended effects. The straightforward solution is temporal separation.

Dose-Separation Strategy

The goal is to let modafinil work during its active window and glycine work during its intended sleep window. Because modafinil's half-life is 12 to 15 hours, timing matters.

Morning Modafinil, Bedtime Glycine

Take modafinil as early in the day as possible. The FDA label recommends a single morning dose (100 to 200 mg for narcolepsy, 200 mg for shift-work disorder taken one hour before the shift) [2]. Taking modafinil before 9 a.m. Means plasma levels will have dropped substantially by 10 p.m.

Take glycine 30 to 60 minutes before bed. The Bannai et al. Trial used 3 g at bedtime and observed reductions in polysomnographic sleep-onset latency and improvements in subjective morning alertness [4].

What About Split Dosing?

Some prescribers split modafinil into a morning and early-afternoon dose (100 mg + 100 mg). If you use a split regimen, ensure the second dose is taken no later than 1 p.m. To maintain adequate clearance before bedtime glycine.

Doses to Discuss with Your Prescriber

| Compound | Typical dose | Timing | |---|---|---| | Modafinil | 100 to 200 mg | Morning (before 9 a.m.) | | Glycine | 3 g | 30 to 60 min before bed |

If insomnia persists despite this schedule, your prescriber may consider lowering the modafinil dose before adding glycine, since addressing the root cause (modafinil-induced arousal at night) is preferable to layering supplements.

Glycine's Effects on Blood Glucose: Relevant for Modafinil Users

A secondary interaction consideration involves blood sugar. Glycine has been shown to potentiate insulin secretion and improve glucose disposal. A 2004 study by Gannon et al. Found that 25 g of glycine co-ingested with 25 g of glucose reduced the glucose area-under-the-curve by approximately 50% in healthy subjects (N = 9) [15]. Even at lower supplement doses (3 to 5 g), glycine stimulates GLP-1 and insulin release [16].

Why This Matters for Modafinil Users

Modafinil does not directly alter glucose metabolism. However, patients who take modafinil for shift-work disorder often have disrupted circadian rhythms and eating patterns, which independently increase insulin resistance risk [17]. If you take both glycine and modafinil, monitor for unexpected drops in fasting blood glucose, especially if you are also on metformin, sulfonylureas, or insulin.

The Endocrine Society's 2022 Clinical Practice Guideline on shift-work disorder notes that "circadian misalignment compounds metabolic risk, and supplement use should be reviewed in the context of glucose-lowering therapy" [18].

Safety Data and Adverse Events

Glycine Safety Profile

Glycine is classified as Generally Recognized as Safe (GRAS) by the FDA for food use [19]. In clinical trials, oral doses of 3 to 9 g per day have been well tolerated with no serious adverse events. The most common side effect is mild gastrointestinal discomfort (nausea, soft stools) at doses above 9 g [20].

Higher doses (15 to 60 g/day) have been studied in schizophrenia trials as an NMDA-modulating strategy. A 2004 Cochrane review of glycine augmentation in schizophrenia (5 trials, N = 141) found no significant safety signals even at these high doses, though mild nausea and vomiting occurred more frequently than with placebo [21].

Modafinil Safety Profile

The most common adverse events with modafinil 200 mg in key trials included headache (34% vs. 23% placebo), nausea (11% vs. 3%), and insomnia (5% vs. 1%) [2]. Rare but serious reactions include Stevens-Johnson syndrome and angioedema, both listed as boxed warnings.

Combined Use

No published case reports document adverse events from concurrent glycine and modafinil use. The Natural Medicines Comprehensive Database does not list this combination as a known interaction [8]. This does not mean zero risk exists. It means the combination has not been formally studied. Treat absence of evidence as a reason for monitoring, not as proof of safety.

Who Should Be Cautious

Not everyone should combine these two compounds without medical guidance. Several populations warrant extra caution.

Patients with Renal Impairment

Glycine is excreted renally. In patients with an eGFR <60 mL/min/1.73 m², glycine clearance may be reduced, leading to accumulation. Modafinil's acid metabolite is also renally cleared [1]. If you have chronic kidney disease, discuss both compounds with your nephrologist before combining them.

Patients on Clozapine or Other NMDA-Modulating Drugs

Glycine's role as an NMDA co-agonist means it could theoretically potentiate or interfere with drugs that also target the NMDA receptor, including clozapine, memantine, and ketamine [12]. If you take any NMDA-active medication alongside modafinil, adding glycine introduces a third variable that your prescriber should evaluate.

Pregnant or Lactating Women

Modafinil is pregnancy category C (now replaced by narrative labeling). Animal studies showed embryotoxicity at high doses [2]. Glycine crosses the placenta, though no teratogenicity has been reported at supplement doses. Neither compound should be used in pregnancy without explicit medical guidance.

Monitoring Recommendations

If you and your prescriber decide to combine glycine and modafinil, a simple monitoring plan reduces risk.

First 2 weeks:

• Keep a sleep diary (time to fall asleep, number of awakenings, subjective morning alertness).
• Note any GI symptoms (nausea, bloating, loose stools).
• Track daytime alertness using a simple 1 to 10 scale at 10 a.m. And 3 p.m.

At 4 weeks:

• Review the sleep diary for trends. If sleep latency has not improved, the glycine dose or timing may need adjustment.
• Check fasting glucose if you are on glucose-lowering medications.

Ongoing:

• Renal function panel (BUN, creatinine, eGFR) annually if glycine dose exceeds 5 g/day.
• Reassess the need for glycine if modafinil is discontinued or the dose changes.

What to Do If You Are Already Taking Both

If you have been taking glycine and modafinil together without problems, there is no urgent reason to stop. Review your timing: morning modafinil and bedtime glycine is the optimal schedule. If you have been taking them simultaneously (for example, both in the morning), consider separating them and tracking whether daytime alertness or nighttime sleep quality changes over the following two weeks.

Report any unusual symptoms (excessive daytime sleepiness despite modafinil, vivid dreams, GI distress, or changes in urine output) to your prescriber. These may signal a dose or timing issue rather than a true drug-supplement interaction.

Prescribers should document glycine use in the patient's medication list, since over-the-counter supplements are frequently omitted from reconciliation and can introduce unrecognized variables into treatment plans.