Elagolix and Relugolix: Oral GnRH Antagonists
GnRH and Endometriosis
75.8% dysmenorrhea response
In the Elaris Phase 3 trials, 75.8% of women taking high-dose elagolix had a clinical response for dysmenorrhea, compared to 19.6% on placebo. Relugolix combination therapy matched this efficacy with better bone safety.
Taylor et al., NEJM, 2017; Giudice et al., Lancet, 2022
Taylor et al., NEJM, 2017; Giudice et al., Lancet, 2022
View as imageEndometriosis treatment changed fundamentally between 2018 and 2024. For decades, hormonal management relied on injectable GnRH agonists (leuprolide, goserelin) that required an initial 10-12 day symptom flare before therapeutic suppression began, and were limited to six months of monotherapy due to bone mineral density loss. Two oral GnRH antagonists, elagolix (Orilissa, FDA-approved 2018) and relugolix combination therapy (Myfembree, FDA-approved 2024 for endometriosis), bypass both limitations. They suppress estrogen within days rather than weeks, avoid the flare-up entirely, and offer dosing flexibility that injectable agonists cannot match. For how the older GnRH agonist approach works and where it falls short, see the pillar article in this cluster.
This article compares the two drugs head-to-head on efficacy, bone safety, and practical use.
Key Takeaways
- Elagolix at 200 mg twice daily produced dysmenorrhea response in 75.8% of women versus 19.6% on placebo in Phase 3 trials, but is limited to 6 months at this dose due to bone mineral density loss (Taylor et al., NEJM, 2017)
- Elagolix at 150 mg once daily produced lower response rates (46.4%) but is approved for up to 24 months, offering a partial-suppression option with better bone safety (Taylor et al., NEJM, 2017)
- Relugolix combination therapy (40 mg relugolix + 1 mg estradiol + 0.5 mg norethindrone) achieved approximately 75% dysmenorrhea response with minimal bone loss over 24 weeks in the SPIRIT Phase 3 trials (Giudice et al., Lancet, 2022)
- Two-year extension data showed sustained pain relief with relugolix combination therapy, with bone mineral density changes remaining within clinically acceptable limits (Becker et al., Human Reproduction, 2024)
- Unlike GnRH agonists, neither elagolix nor relugolix causes an initial flare-up of symptoms, since antagonists directly block the GnRH receptor rather than overstimulating it
Antagonist vs Agonist: Why the Mechanism Matters
GnRH agonists and antagonists both reduce estrogen production, but through opposite pharmacological mechanisms with different clinical consequences.
GnRH agonists (leuprolide, goserelin, nafarelin) continuously stimulate pituitary GnRH receptors until they downregulate and stop responding. This takes 10-12 days, during which estrogen levels actually rise before crashing. The initial surge, called the flare-up, can temporarily worsen endometriosis pain, sometimes severely.
GnRH antagonists (elagolix, relugolix, linzagolix) directly block the GnRH receptor, preventing the natural GnRH signal from reaching the pituitary. Suppression begins within hours of the first dose. There is no flare. There is no surge. Estrogen levels decline in a dose-dependent fashion: lower doses produce partial suppression (estradiol 20-50 pg/mL), while higher doses approach full suppression (estradiol less than 10 pg/mL).
This dose-dependent control is the key advantage of oral GnRH antagonists. Injectable agonists are binary: once injected, you get full suppression after the flare period, and you cannot adjust the dose until the depot formulation wears off (1-3 months). Oral antagonists can be titrated daily, allowing clinicians to find the minimum effective suppression for each patient. For how the same GnRH antagonist mechanism is applied in IVF protocols, the rapid onset and reversibility serve a different clinical purpose.
Elagolix (Orilissa): The First Oral GnRH Antagonist
Phase 3 Clinical Trials (Elaris EM-I and EM-II)
Taylor et al. (2017) published the pivotal Phase 3 trials in the New England Journal of Medicine.[1] Two replicate, double-blind, randomized trials enrolled a combined 1,686 women with surgically diagnosed endometriosis and moderate-to-severe pain. Two doses were tested against placebo over six months.
Lower dose (150 mg once daily):
- Dysmenorrhea responder rate: 46.4% versus 19.6% placebo
- Non-menstrual pelvic pain responder rate: 50.4% versus 36.5% placebo
- Estradiol levels: partial suppression (average 42 pg/mL)
- Approved for up to 24 months of continuous use
Higher dose (200 mg twice daily):
- Dysmenorrhea responder rate: 75.8% versus 19.6% placebo
- Non-menstrual pelvic pain responder rate: 54.5% versus 36.5% placebo
- Estradiol levels: near-complete suppression (average 12 pg/mL)
- Limited to 6 months due to bone mineral density concerns
The dose-response relationship illustrates the tradeoff at the heart of endometriosis treatment: more estrogen suppression produces more pain relief but also more bone loss. At 150 mg daily, elagolix suppresses estrogen enough to reduce endometriotic lesion activity while maintaining sufficient estrogen to protect bone. At 200 mg twice daily, suppression approaches the hypoestrogenic state produced by GnRH agonists, with similar bone consequences.
Long-Term Extension Data
Surrey et al. (2018) published 12-month extension study results.[2] Pain relief was maintained through 12 months of continuous elagolix treatment. Bone mineral density at the lumbar spine decreased by 2.61% (150 mg) and 6.30% (200 mg twice daily) from baseline over 12 months. For the lower dose, bone mineral density partially recovered after treatment discontinuation. For the higher dose, recovery was less complete.
A 2021 clinician's guide synthesized the practical implications of these data, establishing treatment algorithms that consider disease severity, bone risk factors, and patient preference when choosing between the two elagolix doses.[3]
Relugolix Combination Therapy (Myfembree): Built-in Add-Back
The Combination Strategy
Relugolix was designed from the outset as a combination tablet containing three components: relugolix (40 mg, the GnRH antagonist), estradiol (1 mg, for bone and vasomotor protection), and norethindrone acetate (0.5 mg, a progestin to protect the endometrium from estrogen stimulation). This built-in "add-back" therapy addresses the bone safety limitation that constrains elagolix's high dose to six months.
The concept of add-back therapy is not new. Clinicians have prescribed low-dose estrogen alongside GnRH agonists since the 1990s, based on the "estrogen threshold hypothesis": endometriotic tissue requires higher estrogen levels to grow than bone requires to maintain density. By suppressing estrogen fully with the antagonist and then adding back a small amount, the combination keeps estrogen below the threshold that feeds endometriosis but above the threshold that causes bone loss.
Phase 3 Clinical Trials (SPIRIT 1 and SPIRIT 2)
Giudice et al. (2022) published the pivotal SPIRIT trials in The Lancet.[4] Two replicate Phase 3 trials enrolled 1,263 women with endometriosis-associated pain. Relugolix combination therapy was compared to placebo over 24 weeks.
Efficacy results:
- Dysmenorrhea responder rate: approximately 75% versus approximately 27% placebo
- Non-menstrual pelvic pain responder rate: approximately 59% versus approximately 40% placebo
- Dyspareunia (pain during intercourse): significant improvement versus placebo
- Rescue analgesic use: significantly reduced versus placebo
Bone mineral density at 24 weeks:
- Relugolix combination therapy: approximately -0.7% at lumbar spine
- Relugolix alone (without add-back): approximately -2.0% at lumbar spine
Two-Year Extension Data
Becker et al. (2024) published the SPIRIT open-label extension results in Human Reproduction, providing the longest-duration data for any oral GnRH antagonist in endometriosis.[5] Over 2 years of continuous treatment:
- Pain relief (dysmenorrhea and non-menstrual pelvic pain) was maintained throughout
- Bone mineral density changes remained within clinically acceptable limits (approximately -1% at lumbar spine at 2 years)
- Hot flushes and other vasomotor symptoms were minimal
- No new safety signals emerged with extended use
Head-to-Head Comparison
No randomized trial has directly compared elagolix to relugolix combination therapy. The comparison below is cross-trial and should be interpreted with caution.
| Parameter | Elagolix 150 mg QD | Elagolix 200 mg BID | Relugolix CT |
|---|---|---|---|
| Dysmenorrhea response | ~46% | ~76% | ~75% |
| Pelvic pain response | ~50% | ~55% | ~59% |
| Bone loss (6 months) | ~-1.0% | ~-3.1% | ~-0.7% |
| Max approved duration | 24 months | 6 months | 24 months |
| Add-back included | No | No | Yes (built-in) |
| Daily dosing | Once daily | Twice daily | Once daily |
Relugolix combination therapy achieves efficacy comparable to high-dose elagolix while maintaining bone safety closer to low-dose elagolix. The built-in add-back eliminates the need for separate prescriptions.
The Bone Density Tradeoff
Bone mineral density loss is the limiting factor for all hormonal endometriosis treatments that suppress estrogen. For elagolix at 150 mg daily, cumulative bone loss is approximately 2.6% at the lumbar spine over 12 months, with partial recovery after discontinuation.[2] For elagolix at 200 mg twice daily, loss reaches 3.1% at 6 months and 6.3% at 12 months in extension studies, with incomplete recovery.
Relugolix combination therapy limits lumbar spine bone loss to approximately 0.7% at 24 weeks and approximately 1% at 2 years.[5] This is within the range of normal physiological variation and is the primary clinical argument for relugolix combination therapy over elagolix for patients requiring long-term treatment.
Practical Considerations: Choosing Between Elagolix and Relugolix
The choice between elagolix and relugolix combination therapy depends on several clinical factors that the trial data alone cannot resolve.
Disease severity and treatment duration. For women with mild-to-moderate endometriosis pain who may need only a few months of hormonal suppression (for instance, bridging to surgery or managing a flare), elagolix at 150 mg daily offers adequate partial suppression with good bone safety and no additional hormones. For women with moderate-to-severe pain requiring long-term management, relugolix combination therapy provides maximum efficacy (matching elagolix 200 mg) with 2-year safety data supporting extended use.
Bone risk factors. Women with existing osteopenia, family history of osteoporosis, vitamin D deficiency, low body weight, or smoking history face higher baseline bone risk. For these patients, relugolix combination therapy's minimal bone impact (approximately -0.7% at 6 months versus -3.1% for high-dose elagolix) represents a meaningful safety advantage. Elagolix at 200 mg twice daily is explicitly limited to 6 months largely because of bone concerns.
Contraception requirements. Neither elagolix alone nor relugolix combination therapy is a contraceptive. Both reduce fertility during treatment, but pregnancy can still occur. This is a clinical consideration often overlooked in discussions of these drugs: women using oral GnRH antagonists for endometriosis typically need separate contraception, and the interaction between the GnRH antagonist's estrogen suppression and additional hormonal contraceptives requires careful management.
Reversal of fertility. Both drugs are rapidly reversible. Ovulation typically resumes within 4-8 weeks of discontinuation. This is a significant advantage over depot GnRH agonist injections, where suppression can persist for months after the final injection. For women who may want to conceive in the near future, the rapid reversibility of oral antagonists offers more flexibility in treatment planning.
Emerging Peptide Targets in Endometriosis Pain
Research beyond the GnRH axis is identifying new peptide mechanisms in endometriosis. Fattori et al. (2024) demonstrated that CGRP/RAMP1 signaling from sensory nerves drives both endometriosis-associated pain and lesion growth.[6] FDA-approved CGRP-blocking drugs (originally developed for migraine) reduced both pain and lesion size in mouse models, suggesting that future endometriosis treatment may combine GnRH antagonists with targeted pain pathway blockade. For the full landscape of peptide-based approaches to endometriosis, non-hormonal interventions are an active research area.
The Bottom Line
Elagolix and relugolix are both FDA-approved oral GnRH antagonists that treat endometriosis pain by directly blocking pituitary GnRH receptors without the flare-up effect of older injectable agonists. Elagolix offers two dose options: 150 mg daily (46% dysmenorrhea response, 24-month limit) and 200 mg twice daily (76% response, 6-month limit). Relugolix combination therapy matches the higher dose's efficacy (approximately 75% response) while achieving bone safety comparable to the lower dose through built-in estradiol/norethindrone add-back. Two-year data confirms sustained pain relief with minimal bone impact. Cross-trial comparison favors relugolix combination therapy for patients needing long-term maximum-efficacy treatment.