Can I Take Glycine with Prometrium?

What Is Prometrium and Why Do Women Take It?

Prometrium is an FDA-approved oral formulation of micronized progesterone suspended in peanut oil. Because the progesterone is micronized to tiny particles, it absorbs meaningfully through the gastrointestinal tract, unlike older synthetic progestins. Women across several life stages encounter it.

Perimenopausal and Postmenopausal Use

In women using systemic estrogen therapy for menopause symptoms, Prometrium at 200 mg nightly for 12 days per cycle or 100 mg continuously provides endometrial protection. The 2022 Menopause Society hormone therapy position statement affirms that micronized progesterone is the preferred progestogen for women who can take oral therapy, partly because its cardiovascular and breast-cancer risk profile appears more favorable than that of medroxyprogesterone acetate.

Reproductive-Age and Fertility Use

Prometrium is also prescribed during the luteal phase in assisted reproductive technology cycles and for luteal-phase deficiency. ASRM guidelines on luteal-phase support acknowledge vaginal progesterone as preferred for IVF, though oral micronized progesterone is used when vaginal routes are not tolerated.

Early Pregnancy Use

Some practitioners prescribe Prometrium to support early pregnancy in women with recurrent miscarriage or luteal insufficiency. The PROMISE trial (Coomarasamy et al., NEJM 2015) found no significant reduction in live birth rates with vaginal progesterone in unselected recurrent-miscarriage patients, though the subsequent PRISM trial (Coomarasamy et al., NEJM 2019) showed benefit in women with early pregnancy bleeding and previous miscarriage.

What Is Glycine and Why Are Women Using It?

Glycine is the simplest amino acid and is classified as conditionally essential. Your body synthesizes roughly 3 g per day, but dietary and supplemental intake often adds considerably more. Women reach for glycine for three main reasons: sleep quality, skin and joint collagen support, and metabolic health.

Sleep and Neurological Effects

A 2012 randomized crossover trial by Bannai et al. Published in Sleep and Biological Rhythms showed that 3 g of glycine taken 1 hour before bed reduced subjective fatigue, improved sleep onset, and shortened time to slow-wave sleep in adults who self-reported unsatisfactory sleep. The mechanism involves glycine acting as an inhibitory neurotransmitter in the brainstem and spinal cord and causing mild peripheral vasodilation that lowers core body temperature, a known sleep-onset signal.

Collagen and Skin Health

Glycine makes up roughly one-third of collagen's amino acid sequence. Supplemental glycine combined with proline and vitamin C can increase collagen synthesis in dermal fibroblasts. A 2021 review in Nutrients by Martínez-Puig et al. concluded that collagen peptide supplementation, of which glycine is a primary constituent, reduced skin wrinkle depth in RCTs. This is particularly relevant for postmenopausal women, who lose an estimated 30% of skin collagen in the first five years after menopause.

Metabolic and Glycemic Effects

Glycine stimulates glucagon-like peptide-1 (GLP-1) secretion from intestinal L-cells and has demonstrated insulin-sensitizing properties in several small human studies. A 2016 study in Diabetes Care by Gannon et al. found that 5 g of glycine taken with a glucose load reduced the postprandial glucose excursion. This matters specifically for women with PCOS, who carry a high prevalence of insulin resistance.

Is There a Pharmacokinetic Interaction Between Glycine and Prometrium?

No. The two compounds travel entirely different metabolic pathways, and no pharmacokinetic interaction has been documented in the primary literature or flagged in major interaction databases.

How Prometrium Is Metabolized

Prometrium is absorbed in the small intestine and undergoes extensive first-pass hepatic metabolism via CYP3A4 and CYP2C19. Peak serum progesterone occurs roughly 2 to 4 hours after an oral dose. Metabolites include allopregnanolone and pregnanolone, neuroactive steroids that bind GABA-A receptors, which is the primary explanation for the sedation women notice.

How Glycine Is Processed

Glycine is not a substrate of CYP450 enzymes. It is transported across the gut wall via sodium-dependent glycine transporters (GlyT1 and GlyT2) and cleared renally or catabolized to pyruvate via the glycine cleavage system. There is no shared hepatic metabolic pathway with progesterone, no protein-binding competition, and no impact of glycine on CYP3A4 or CYP2C19 activity at any dose studied in humans.

The bottom line: glycine will not change Prometrium blood levels. You do not need to time them apart for pharmacokinetic reasons.

The Real Concern: Additive Sedation at Bedtime

This is where clinical judgment matters. Both compounds promote sleep through distinct but parallel mechanisms, and that overlap has practical implications for you.

Prometrium's neuroactive metabolites, particularly allopregnanolone, are positive allosteric modulators of GABA-A receptors, the same receptor family targeted by benzodiazepines and alcohol. This is why the Prometrium prescribing information specifically recommends taking the capsule at bedtime. In clinical practice, women on 200 mg nightly often report notable grogginess the following morning, and a subset describe dizziness or impaired coordination if they take it earlier in the evening and try to stay active.

Glycine's sedative mechanism is separate. It lowers core body temperature via peripheral vasodilation and acts directly on NMDA receptors and glycinergic interneurons in the brainstem. Bannai et al. (2012) used only 3 g and documented measurable reduction in sleep latency with no hangover effect in healthy adults. However, no study has tested glycine alongside a GABA-A-active progesterone metabolite, so the combined sedative depth is extrapolated rather than directly measured. This is an evidence gap you deserve to know about.

Practical Guidance on Timing and Dose

• If you take Prometrium at bedtime (the standard recommendation), adding glycine at the same time is unlikely to cause dangerous sedation in healthy women at typical doses (2 to 3 g).
• At higher glycine doses (8 to 10 g), the additive effect on sleep depth is untested alongside Prometrium. Starting at 2 g and assessing your response before increasing makes sense.
• Do not drive or operate machinery in the hours following a bedtime dose of Prometrium regardless of whether you add glycine.
• Women on 100 mg continuous Prometrium report less sedation than those on 200 mg cyclic dosing. Your dose matters when estimating combined effect.

Glycine and PCOS: A Female-Specific Consideration

Women with PCOS occupy a clinically distinct position when combining these two compounds. Many women with PCOS prescribed Prometrium are using it to induce withdrawal bleeds, protect the endometrium from unopposed estrogen produced by anovulatory cycles, or support a luteal phase in a fertility protocol.

PCOS also carries a 50 to 70% prevalence of insulin resistance. Glycine's GLP-1-stimulating and insulin-sensitizing properties could be genuinely useful in this group, not merely neutral. However, women with PCOS who are also taking metformin should be aware that the glycemic signals from multiple agents can interact, not because of pharmacokinetic overlap, but because each lowers postprandial glucose through partially distinct mechanisms.

A practical framework for women with PCOS considering both:

1. Sleep support goal: 2 to 3 g glycine at bedtime alongside 200 mg Prometrium on the days it is prescribed. Monitor morning alertness for the first week.
2. Metabolic/insulin-resistance goal: 5 g glycine with the largest meal of the day, separate from the bedtime Prometrium dose. This avoids any additive sedation concern and targets the glycemic benefit at the time it is most relevant.
3. Collagen/skin goal: Glycine (or a collagen peptide providing 2.5 to 5 g glycine-rich peptides) at any time of day, away from bedtime if you are sensitive to the combined sedative load.

Pregnancy and Lactation Safety

Prometrium in Pregnancy

Prometrium is not classified under the old letter-category system by the FDA for new approvals, but its prescribing label carries human data from early-pregnancy support use. Exogenous progesterone is a naturally occurring hormone essential for pregnancy maintenance. The PRISM trial used vaginal progesterone at 400 mg twice daily in the first trimester, and the drug has a long history of use for luteal-phase support. No pattern of fetal malformation has been attributed to natural micronized progesterone in observational studies, unlike some older synthetic progestins. Prometrium capsules contain peanut oil; if you have a peanut allergy, alert your prescriber before use during pregnancy.

If you are trying to conceive or are in early pregnancy and your provider has prescribed Prometrium, do not stop it without guidance. Stopping abruptly can withdraw progesterone support at a stage the pregnancy may depend on it.

Prometrium in Lactation

Progesterone transfers into breast milk in small amounts. LactMed, the NIH's database of drugs and lactation, notes that exogenous progesterone has not been studied thoroughly in lactating women but that natural progesterone's milk:plasma ratio is low and systemic infant exposure is expected to be minimal. The clinical decision to use Prometrium postpartum should weigh the indication (typically endometrial protection with HRT, less common in early postpartum), milk production concerns, and provider judgment.

Glycine in Pregnancy

Glycine is a non-essential amino acid found in meat, legumes, and bone broth and is present in normal dietary intake throughout pregnancy. It is conditionally essential during periods of rapid growth such as fetal development. No human RCT of supplemental glycine specifically in pregnant women has established a definitive safe upper dose, so extrapolation from dietary intake is the current basis for reassurance. At the doses used for sleep (2 to 3 g) or collagen support (up to 10 g), glycine is generally regarded as low-risk, but direct pregnancy safety data are thin. Discuss supplementation with your obstetrician if you are pregnant.

Glycine in Lactation

No specific lactation data on supplemental glycine exist in LactMed or the major pharmacovigilance databases. Given its status as a naturally occurring dietary amino acid, risk is considered low, but documented evidence is absent. Again, this is an evidence gap. If you are breastfeeding and want to take glycine for sleep, a brief conversation with your provider is appropriate.

Contraception Note

Prometrium prescribed for HRT in postmenopausal women does not provide contraception. Micronized progesterone at typical HRT doses does not reliably suppress ovulation. If you are perimenopausal and still ovulating, use a reliable contraceptive method alongside HRT. ACOG Practice Bulletin 141 notes that perimenopausal women should not assume HRT eliminates pregnancy risk.

Who This Combination May Be Right For (and Who Should Be Cautious)

Likely Fine With Standard Monitoring

• Postmenopausal women on continuous 100 mg Prometrium adding 2 to 3 g glycine at bedtime for sleep or skin support
• Perimenopausal women on cyclic 200 mg Prometrium who are not reporting excessive morning sedation
• Women with PCOS taking Prometrium for withdrawal bleeds who want glycine for insulin sensitization, provided the glycine is taken with meals rather than at bedtime

Reason for Extra Caution

• Women already experiencing significant sedation or next-morning grogginess on Prometrium alone. Adding any additional CNS-depressant effect, even mild, may worsen function.
• Women concurrently taking other GABA-active compounds such as benzodiazepines, gabapentin, or alcohol. Glycine's additive contribution becomes harder to isolate in this context.
• Women with a history of hypoglycemia or on multiple glucose-lowering agents. At 5 to 10 g doses, glycine's GLP-1 and insulin-sensitizing effects may incrementally lower postprandial glucose, which is generally beneficial but worth monitoring.
• Women with severe renal impairment. Glycine is cleared renally, and accumulation could occur, though this is a theoretical concern at supplement doses.

Monitoring and What to Do If You Are Already Taking Both

If you are currently combining glycine and Prometrium without any guidance, the news is reassuring: a serious adverse interaction is not expected based on the available pharmacology. Still, a few practical steps are worth taking.

Assess your sedation level. On the mornings after taking both, rate your alertness on a simple 1-to-5 scale for one week. If you consistently score below 3, reduce the glycine dose or shift it to a different time of day.

Check your morning glucose if you have PCOS or insulin resistance. Glycine at doses above 5 g taken with the evening dose of Prometrium is an untested combination from a glycemic standpoint. A fasting fingerstick or continuous glucose monitor trace for a week gives useful data.

Tell your prescriber. This is not a situation requiring urgent action, but your provider should know all supplements you take. The Menopause Society recommends full supplement disclosure as part of routine HRT follow-up because herb-drug interactions in the menopause population are common and frequently undisclosed.

Review at your next visit. There is no need to stop either compound emergently. Bring your one-week sedation log and any glucose data to your next appointment. Your provider can then make an informed decision about whether to adjust timing, dose, or monitoring frequency.

The Evidence Gap: What We Do Not Know

Women deserve directness about the limits of the data here. No randomized controlled trial has specifically studied the pharmacodynamic interaction between micronized progesterone and supplemental glycine in women. The sedation concern is extrapolated from:

• Prometrium's known allopregnanolone-mediated GABA-A activity, confirmed in human pharmacokinetic studies by Simon et al. Published in Fertility and Sterility
• Glycine's sleep-promoting effects established in Bannai et al. (2012) and Inagawa et al. (2006) in Sleep and Biological Rhythms
• General pharmacodynamic principles of additive CNS-depressant effects

The glycemic data are similarly extrapolated from short-term metabolic studies in non-pregnant adults, not in women specifically on progesterone therapy. Women have been historically underrepresented in pharmacokinetic trials involving amino acid supplements, and sex-specific data on glycine's GLP-1 effects across the menstrual cycle do not exist in the peer-reviewed literature as of this writing.

What exists is enough to make a reasonable clinical judgment. It is not enough to issue definitive dosing recommendations specific to this combination. Your prescriber's knowledge of your full clinical picture fills the gap that the research has not yet addressed.