Can I Take Glycine with Lantus? What the Evidence Says

Insulin glargine (brand name Lantus, also sold as Basaglar and Toujeo in different concentrations) is a long-acting basal insulin analog dosed once daily by subcutaneous injection. Glycine is a non-essential amino acid sold as an over-the-counter supplement, typically marketed for sleep, collagen support, or general metabolic health, and taken orally in doses ranging from 1 g to 15 g per day.

There is no known pharmacokinetic interaction between oral glycine and injected insulin glargine. Glycine does not compete with insulin glargine for absorption, transport proteins, or hepatic metabolism, so it cannot change how fast or how long Lantus acts. The question worth asking is different: because glycine has its own modest, dose-dependent glucose-lowering effect, does that effect add to Lantus's action enough to matter? At typical supplement doses of 1 to 3 g per day, the added effect appears small. At the higher doses used in research on glycine and type 2 diabetes (10 to 15 g per day), the combined glucose-lowering effect becomes large enough to warrant closer monitoring and clinician involvement.

What Is Established, What Is Plausible, and What Is Not

Established: Glycine and insulin glargine use unrelated absorption and clearance pathways. Glycine is absorbed through intestinal amino acid transporters and cleared largely through the hepatic glycine cleavage system; insulin glargine is absorbed slowly from a subcutaneous depot and degraded by tissue proteases, not by cytochrome P450 enzymes. There is no plausible mechanism by which glycine would alter Lantus's onset, duration, or peak.

Plausible but not fully quantified for this specific combination: Small human trials have reported that oral glycine, at doses well above what most supplement users take for sleep, can blunt a post-meal glucose rise, and that sustained higher-dose glycine (in the range of 15 g/day) may modestly lower fasting glucose and HbA1c in people with type 2 diabetes over months of use. These studies were not designed to test glycine on top of basal insulin therapy specifically, so the size of any additive hypoglycemia risk in Lantus users has not been directly measured. The exact effect sizes cited in older secondary sources for this topic should be verified against the original trial reports before being treated as precise numbers.

Not established: There are no published trials of glycine supplementation specifically in patients using insulin glargine that quantify hypoglycemia rates, optimal monitoring intervals, or insulin dose-adjustment thresholds. Recommendations below on monitoring frequency and dose thresholds are reasoned extrapolations from general pharmacodynamic principles and the FDA-defined hypoglycemia threshold, not outcomes from a dedicated interaction study.

Lantus Already Contains Glycine, But at a Trace Amount

Insulin glargine injection (Lantus) lists glycine as an inactive ingredient, used as a tonicity agent and stabilizer in the acidic pH 4.0 formulation, at a concentration of approximately 3.6 mg per mL, according to the drug's prescribing information. A typical injected dose delivers a few milligrams of glycine subcutaneously. This is one to three orders of magnitude below an oral supplement dose (1,000 to 15,000 mg) and has no measurable metabolic effect on its own. The presence of glycine on the Lantus label is a formulation detail, not evidence of a meaningful interaction with oral glycine supplements.

Why There Is No Pharmacokinetic Conflict

Insulin glargine forms microprecipitates in subcutaneous tissue that dissolve gradually, producing a relatively flat insulin level over roughly 20 to 24 hours. Nothing about oral glycine absorption, which happens in the small intestine within an hour or two of ingestion, intersects with that subcutaneous depot process. Glycine does not bind plasma proteins in a way that would displace insulin from a carrier, and insulin glargine acts through the insulin receptor rather than through a metabolic pathway glycine could inhibit or induce. Because of this, no dose-separation window between an oral glycine supplement and a Lantus injection is pharmacokinetically necessary.

The Real Question: Does the Glucose-Lowering Effect Stack?

Both glycine and insulin glargine lower blood glucose, through different mechanisms. Small published trials in healthy volunteers have described glycine blunting the glucose response to a carbohydrate load when given in relatively large single doses (tens of grams per kilogram of body weight in some protocols), and separate trials in people with type 2 diabetes have described modest reductions in fasting glucose and HbA1c with sustained multi-gram daily dosing over several months. These findings support a real, dose-dependent glucose-lowering effect from glycine itself, independent of any insulin. The magnitude reported in these specific trials should be confirmed against the primary publications before being used to make an individual dosing decision, since secondary summaries of this literature vary and some numbers attached to it online are not consistently sourced.

Because Lantus is titrated to bring fasting glucose into a target range, adding an independent glucose-lowering agent on top of a stable Lantus dose can push glucose lower than intended. This is the same principle that applies to starting any new glucose-lowering supplement or medication in a patient already on basal insulin: the practical risk is additive hypoglycemia, not a change in how Lantus itself behaves.

Evidence-Status Interaction Assessment: Glycine + Lantus

Sleep Dosing: The Most Common Real-World Scenario

Most people who take glycine while on Lantus are doing so for sleep, typically around 3 g before bed. Small trials in healthy volunteers have reported that this dose improves subjective sleep quality and reduces the time it takes to fall asleep, without reporting a large next-morning glucose disruption. At this dose, the glucose-lowering effect of glycine is expected to be modest based on the general dose-response pattern seen in the available research, but it does overlap in timing with the onset phase of a bedtime Lantus injection.

A practical precaution for anyone combining bedtime glycine with a bedtime Lantus dose, particularly those who already run fasting glucose values below 90 mg/dL or who have a history of nocturnal hypoglycemia, is to check a middle-of-the-night glucose value for the first several nights after starting glycine. This does not require a formal dose-separation schedule; it is a monitoring step, not a pharmacokinetic requirement.

Collagen Supplements: A Lower-Concern Source of Glycine

Collagen peptide supplements are another common source of glycine exposure, typically delivering roughly 2 to 3 g of glycine per 10 g serving. Because collagen is usually taken in the morning with coffee or after exercise rather than at bedtime, its glucose-lowering window is less likely to overlap with the onset phase of a bedtime Lantus dose. This timing difference lowers the practical concern relative to bedtime glycine use, though the underlying glycine dose is similar.

Higher-Dose Glycine for Metabolic Support: Where More Caution Applies

Research on glycine specifically in people with type 2 diabetes has generally used doses in the 10 to 15 g/day range, well above typical sleep or collagen dosing. Trials in this range have reported reductions in fasting glucose and HbA1c along with changes in inflammatory markers over several months of use. If those effect sizes hold up under verification, a reduction of this magnitude in a patient already titrated to target on Lantus could plausibly lower fasting glucose enough to approach or cross the hypoglycemia threshold, defined as blood glucose below 70 mg/dL.

Patients considering glycine at this dose range alongside Lantus should not start on their own. This is a scenario that benefits from the same caution used when adding any new glucose-lowering agent to an existing insulin regimen: baseline glucose check, close monitoring during the first several weeks, and a clear plan with the prescribing clinician for adjusting the Lantus dose if fasting glucose trends down.

Monitoring by Glycine Dose

• 1 to 3 g/day (typical sleep dose or one collagen serving): continue routine Lantus monitoring, and check an extra fasting glucose on days 3 and 7 after starting glycine.
• 3 to 10 g/day (combination supplements or higher collagen intake): check fasting glucose daily for the first week; if it falls more than 15 mg/dL below your pre-glycine baseline on two or more days, contact your clinician.
• 10 to 15 g/day (research-level metabolic-support dosing): treat this the way you would treat starting a new glucose-lowering medication. Increase glucose checks to several times a day for the first few weeks, keep fast-acting glucose available, and involve your prescribing clinician before starting.

Renal Function and Glycine Clearance

Glycine is cleared renally. In patients with reduced kidney function, slower glycine clearance could plausibly raise plasma glycine levels and increase its glucose-lowering effect, though dedicated data describing this dose-response relationship in people with chronic kidney disease are limited. A reasonable, conservative approach is to start at the lowest effective glycine dose and monitor glucose more closely if estimated GFR is below 60 mL/min/1.73 m², and to avoid glycine supplementation altogether if eGFR is below 30 until a clinician has reviewed the individual situation.

Safety Profile of Glycine Itself

Glycine has a generally favorable safety record in the doses studied, with mild nausea or loose stools as the most commonly reported side effects, usually resolving within the first week. No consistent liver toxicity signal has been reported in human supplementation studies at doses up to 15 g/day for several months. Routine liver monitoring is not required based on glycine use alone.

When to Avoid Combining Glycine with Lantus

• Hypoglycemia unawareness. If you cannot reliably sense low blood sugar, adding any independent glucose-lowering supplement increases risk, and glycine should not be added without close clinician supervision.
• Severe renal impairment (eGFR below 30). Predictable dosing data in this population do not exist; avoid glycine supplementation until a clinician has reviewed your case.
• Concurrent magnesium glycinate use. Magnesium glycinate delivers glycine as the chelating agent, typically 2 to 3 g per serving. Combined with a standalone glycine supplement, total glycine intake can be higher than a patient realizes. Add up glycine from all sources, including collagen, before assuming you are in the low-dose range.

Outside these situations, glycine at typical supplement doses appears compatible with Lantus therapy, with the monitoring outlined above.

If You Are Already Taking Both

If you have combined glycine and Lantus without problems, you likely do not need to change anything. Confirm three things: your fasting glucose has stayed stable since starting glycine, you have not had new hypoglycemia symptoms (shakiness, sweating, confusion, or a measured glucose below 70 mg/dL), and you have not recently increased your glycine dose. If all three hold, continue your current regimen. If you plan to raise your glycine dose, especially into the 10 g/day-and-above range, apply the monitoring steps above and loop in your prescribing clinician beforehand.

When to Seek Urgent Care

Seek urgent medical attention for confusion, loss of consciousness, seizure, or an inability to safely treat a low blood glucose reading with oral carbohydrate. These are signs of severe hypoglycemia and are a medical emergency regardless of whether glycine is a contributing factor.

Note on sourcing: the studies on glycine's glucose-lowering and sleep effects described above are summarized from small human trials referenced in prior versions of this article. Their specific journal citations and PMIDs could not be independently confirmed for this revision and have been removed rather than carried forward inaccurately. Exact effect sizes, sample sizes, and dosing protocols should be verified against the primary literature before being used in patient-facing dosing guidance.