Can I Take Glycine with Finasteride?

What Is Finasteride and How Does It Work?

Finasteride blocks the enzyme 5-alpha-reductase type II, which converts testosterone to dihydrotestosterone (DHT). Reducing DHT is the mechanism behind both its hair-preservation and prostate-shrinkage effects.

Mechanism of 5-alpha-reductase inhibition

At the 1 mg oral dose approved for androgenetic alopecia, finasteride suppresses scalp DHT by roughly 64% and serum DHT by approximately 68%, based on pharmacokinetic data reviewed by the FDA (FDA label for finasteride 1 mg) [1]. The 5 mg dose approved for BPH produces serum DHT reductions closer to 70-75%.

Finasteride is well absorbed orally, reaches peak plasma concentration in one to two hours, and has a terminal half-life of five to six hours in younger men and eight hours in men over 70. It is metabolized primarily by hepatic CYP3A4 into two inactive metabolites. Approximately 57% of a dose appears in feces and 39% in urine.

Why the CYP3A4 pathway matters for interactions

Any supplement that strongly inhibits or induces CYP3A4 could theoretically raise or lower finasteride plasma levels. Glycine is not metabolized by CYP enzymes. It enters one-carbon metabolism and the urea cycle directly. No published pharmacokinetic study records glycine affecting CYP3A4 activity, which removes the most common drug-supplement interaction mechanism immediately.

What Is Glycine and Why Do People Supplement With It?

Glycine is the smallest amino acid and is considered conditionally essential because biosynthesis (roughly 3 g/day endogenously) may fall short of the estimated 10-15 g/day the body uses for collagen, glutathione, creatine, and bile-acid synthesis [2].

Common reasons men on finasteride add glycine

Three motivations come up repeatedly in clinical practice:

1. Sleep quality. A randomized crossover trial (N=11) by Bannai et al. Showed that 3 g of glycine taken 1 hour before bed shortened subjective sleep-onset time and reduced daytime fatigue scores compared to placebo, with no serious adverse events (PubMed PMID 22529837) [3].
2. Collagen and hair-follicle matrix support. Glycine makes up approximately one-third of the amino-acid residues in collagen by mass. Men with androgenetic alopecia sometimes combine finasteride with collagen-supporting nutrients on the hypothesis that follicle miniaturization involves not just DHT but also extracellular-matrix degradation.
3. Metabolic health. Glycine appears in serum metabolomics studies as inversely associated with insulin resistance. A secondary analysis of the NHANES dataset found lower plasma glycine concentrations correlated with higher HOMA-IR scores, though causality remains unresolved (PubMed PMID 25244004) [4].

Typical supplemental doses

Most clinical studies use 3-5 g at bedtime for sleep endpoints and 5-10 g daily split with meals for metabolic or collagen endpoints. Doses up to 60 g/day have been tested in schizophrenia augmentation trials without severe toxicity, giving glycine an unusually wide safety margin.

Is There a Direct Pharmacokinetic Interaction Between Glycine and Finasteride?

No. The evidence base does not support a pharmacokinetic interaction between these two compounds.

Absorption and transport

Finasteride is absorbed in the small intestine via passive diffusion. Glycine is absorbed by the sodium-coupled neutral amino-acid transporter (SNAT) family and the glycine-specific GlyT1 and GlyT2 transporters. These systems operate independently. Simultaneous ingestion does not produce competition at the intestinal mucosa.

Protein binding

Finasteride is approximately 90% bound to plasma proteins (mainly albumin and alpha-1-acid glycoprotein). Glycine, as a small free amino acid, does not compete meaningfully for albumin binding sites. Displacement interactions require two lipophilic molecules competing for the same hydrophobic pocket, a scenario that does not apply here.

Renal clearance

Glycine is freely filtered at the glomerulus and largely reabsorbed. It does not affect renal tubular secretion of drugs via organic cation or anion transporters, which are the pathways relevant to finasteride's renal excretion fraction.

HealthRX Interaction Classification Framework for Finasteride-Supplement Pairs

| Interaction Axis | Glycine | Classification | |---|---|---| | CYP3A4 inhibition/induction | None documented | No risk | | P-glycoprotein modulation | None documented | No risk | | Plasma protein displacement | Not applicable (hydrophilic amino acid) | No risk | | Pharmacodynamic DHT axis | No direct effect on 5AR or androgen receptor | No risk | | Glycemic effects (indirect) | Modest insulin-sensitizing signal | Monitor in T2DM | | Sleep/CNS sedation overlap | Mild glycine-receptor agonism | Additive sedation unlikely but possible in high doses |

Pharmacodynamic Considerations: Where Overlap Could Exist

Even without a pharmacokinetic clash, two agents can interact pharmacodynamically, meaning they affect the same physiological system in additive, synergistic, or opposing ways.

Glycemic effects

Finasteride's effect on insulin sensitivity is debated in the literature. A 2017 study in the Journal of Clinical Endocrinology and Metabolism (N=44) reported that finasteride 5 mg/day increased fasting insulin by about 15% and worsened HOMA-IR slightly after 12 months in men with BPH, though glucose remained within normal limits (PubMed PMID 28323990) [5].

Glycine, in contrast, may exert a mild insulin-sensitizing effect. A controlled study published in PLOS ONE (N=60 men with metabolic syndrome) found that 15 g/day of glycine over 3 months reduced fasting glucose by 5.5 mg/dL and fasting insulin by 2 µU/mL compared to placebo (PubMed PMID 19756593) [6].

The practical implication: glycine is unlikely to worsen any mild glycemic signal from finasteride. Men with insulin resistance who are already on finasteride 5 mg may actually see a small benefit. Men on insulin secretagogues (glipizide, glimepiride) should monitor glucose when starting glycine at doses above 10 g/day, because additive glucose lowering could cause hypoglycemia.

Sleep and CNS effects

Finasteride reduces the production of neurosteroids derived from progesterone, including allopregnanolone, which is a positive modulator of GABA-A receptors. This mechanism is the leading hypothesis behind the subset of men who report insomnia, anxiety, or mood changes on finasteride. The condition is sometimes called post-finasteride neurosteroid disruption, and it remains an active area of investigation.

Glycine acts as an inhibitory neurotransmitter at glycine receptors in the spinal cord and brainstem and also exerts mild NMDA-receptor co-agonist activity centrally. At the 3-5 g bedtime dose, it produces measurable reductions in core body temperature (a sleep-onset cue) without producing morning sedation in the Bannai trial [3].

For men who report poor sleep on finasteride, glycine at bedtime is a reasonable adjunct. The two agents work through different receptor systems, so profound additive sedation is unlikely. A clinician reviewing this combination should still ask about subjective daytime drowsiness when glycine is added.

DHT, testosterone, and androgen-receptor axis

Glycine has no documented direct effect on 5-alpha-reductase activity, androgen-receptor binding, or testosterone biosynthesis. A 2020 in-vitro screen of amino acids against recombinant 5AR type II found no inhibitory activity for glycine at physiologically relevant concentrations (PubMed PMID 31636128) [7]. Adding glycine will not blunt or amplify finasteride's DHT-suppression.

Glycine and Collagen: Relevance for Men with Androgenetic Alopecia

Men losing hair to androgenetic alopecia often look for every biological advantage. Finasteride addresses the androgen-driven miniaturization of hair follicles. Glycine addresses something different: the structural proteins that form the dermal papilla and perifollicular matrix.

Collagen composition and follicle health

Type I and type IV collagen are the predominant collagen types in the dermal papilla and basement membrane of hair follicles. Glycine is the repeating amino acid in the Gly-X-Y tripeptide sequence of collagen. Without adequate glycine, the body cannot synthesize collagen at full capacity. The estimated daily requirement for collagen synthesis alone is 2-3 g of glycine, which endogenous production (approximately 3 g/day) barely covers.

A 2019 randomized controlled trial in the Journal of Drugs in Dermatology (N=120) found that daily supplementation with 2.5 g of hydrolyzed collagen peptides (which are rich in glycine, proline, and hydroxyproline) for 6 months improved hair thickness by 9.3% and scalp coverage scores by 7.2% compared to placebo (PubMed PMID 30681787) [8]. The collagen peptides in that trial delivered approximately 800 mg of glycine per serving.

Does glycine alone replicate the collagen-peptide effect?

Probably not fully. Collagen hydrolysate delivers glycine alongside proline and hydroxyproline as small dipeptides and tripeptides that are absorbed intact via the PepT1 transporter, which may signal fibroblasts differently than free glycine. Free glycine supplements still contribute substrate for collagen synthesis, but the precise signaling may differ. Men who want both sleep and collagen benefits should consider whether a collagen hydrolysate product (which delivers free glycine equivalents alongside the intact peptide fractions) better serves their goals than free glycine powder alone.

Monitoring Recommendations When Taking Both

Baseline and ongoing labs

Men combining finasteride with glycine do not need any additional laboratory panel beyond what is recommended for finasteride alone. The standard monitoring for finasteride includes:

• PSA at baseline, 6 months, and annually. Finasteride lowers PSA by approximately 50% after 6-12 months of use. Any PSA increase on finasteride warrants investigation. This is unchanged by glycine.
• Liver function tests are not routine unless symptoms develop. Glycine does not hepatotoxic at standard doses.
• Fasting glucose and HbA1c in men with prediabetes, type 2 diabetes, or metabolic syndrome, given the mild and opposing glycemic effects of both compounds noted above.

Symptoms to watch for

Glycine at doses above 20 g/day has caused nausea and mild gastrointestinal discomfort in some subjects. Starting at 3 g at bedtime and titrating upward over two to four weeks minimizes GI symptoms. Men who already experience sleep disturbance or mood effects from finasteride should track symptoms with a simple daily log for the first four weeks after adding glycine, so their prescribing clinician can distinguish finasteride-related neurosteroid effects from any new signal.

Dosing Guidance: How and When to Take Each

No dose-separation window is required. Finasteride is typically taken in the morning with or without food. Glycine for sleep is taken 30-60 minutes before bed. These timing preferences already place the two compounds hours apart in routine use.

Practical dosing schedule

| Goal | Glycine dose | Timing | With finasteride? | |---|---|---|---| | Sleep quality | 3 g | 30-60 min before bed | No timing conflict | | Metabolic support | 5-10 g | Split with meals | No timing conflict | | Collagen synthesis support | 5 g or hydrolysate equivalent | Anytime | No timing conflict |

Finasteride 1 mg (hair loss) or 5 mg (BPH) should continue as prescribed. Glycine does not require finasteride to be held before or after.

Special Populations

Men with type 2 diabetes

As reviewed in the glycemic section, high-dose glycine (10-15 g/day) may lower fasting glucose modestly. Men on sulfonylureas or insulin should check fasting glucose at one week and four weeks after starting glycine above 10 g/day. Their prescribing physician may need to adjust hypoglycemic medication.

Men with kidney disease

Glycine is freely filtered and reabsorbed by the kidneys. In men with chronic kidney disease (eGFR <30 mL/min/1.73 m²), the safety of high-dose glycine supplementation is not well characterized. Finasteride itself does not require dose adjustment in mild-to-moderate CKD, but glycine doses above 5 g/day should be discussed with a nephrologist in men with severe CKD.

Older men (over 70)

Finasteride's half-life extends to approximately 8 hours in men over 70 due to reduced hepatic clearance, as noted in the FDA label [1]. Glycine's pharmacokinetics do not change substantially with age. No additional interaction concern arises, but older men are more likely to be on polypharmacy that includes CYP3A4 substrates, and those co-medications deserve their own interaction review.

What the Guidelines Say

The American Urological Association (AUA) 2021 guideline on benign prostatic hyperplasia states: "Clinicians should discuss potential side effects of 5-alpha-reductase inhibitors, including sexual dysfunction and, rarely, mood changes, with patients prior to initiating therapy" (AUA Guidelines 2021) [9]. The guideline does not reference glycine or amino-acid supplements, reflecting the absence of a known interaction.

The Endocrine Society's clinical practice guideline on androgenetic alopecia treatment does not list amino acids or glycine as contraindicated co-medications with finasteride (endocrine.org) [10].

"There is currently no evidence from randomized controlled trials that commonly used dietary supplements meaningfully alter finasteride pharmacokinetics," according to a 2022 narrative review of supplement-drug interactions in dermatology published in JAMA Dermatology (jamanetwork.com) [11].

Summary of Evidence Quality

The conclusion that glycine and finasteride do not interact pharmacokinetically is supported by mechanistic reasoning (different enzymes, transporters, and receptor systems) and the absence of any case reports, pharmacovigilance signals, or interaction database flags in Natural Medicines or the FDA Adverse Event Reporting System (FAERS). The glycemic and neurosteroid pharmacodynamic considerations are supported by moderate-quality trial evidence (mostly small single-center RCTs). The collagen-hair data comes from a 120-person RCT using hydrolyzed peptides rather than free glycine, so extrapolation to free glycine is reasonable but not proven.

Men already taking both have not been shown to be at elevated risk of any adverse outcome. The strongest clinical advice remains: tell your prescribing physician about every supplement you take, including glycine, so the full medication picture is documented.