Head-to-head clinical analysis & difference comparison: details on mechanism of action, dosing, half-life, interactions, and maternal-fetal safety.
LEVONEST vs ALYACEN 1/35
Clinician-reviewed, head-to-head comparison of mechanism, dosing, pharmacokinetics, and safety profiles.
Last clinically reviewed: July 2026 · OpiCalc Medical Review Team
Levonorgestrel is a synthetic progestin that inhibits ovulation by suppressing luteinizing hormone (LH) surge, alters cervical mucus to impede sperm penetration, and induces endometrial changes that inhibit implantation.
Combination hormonal contraceptive: ethinyl estradiol suppresses gonadotropin release via negative feedback on hypothalamic-pituitary axis; norethindrone induces progestational effects including cervical mucus thickening and endometrial changes, inhibiting ovulation and sperm penetration.
Emergency contraception (FDA-approved),Contraception (as a progestin-only pill or in combination with estrogen),Off-label: treatment of menorrhagia, endometriosis, and as a component of hormone replacement therapy
Prevention of pregnancy
One tablet (levonorgestrel 1.5 mg) orally as a single dose within 72 hours of unprotected intercourse.
One tablet (norethindrone 1 mg and ethinyl estradiol 35 mcg) orally once daily for 21 consecutive days, followed by 7 days of placebo or no tablets.
The terminal elimination half-life is approximately 24-30 hours. This relatively long half-life supports once-daily dosing and allows for stable plasma concentrations within 5-7 days of continuous use.
Norethindrone: 8-11 hours (terminal); ethinyl estradiol: 10-20 hours (terminal). The half-life supports once-daily dosing for oral contraceptive efficacy.
Primarily hepatic via CYP3A4; undergoes reduction and conjugation; excreted in urine and feces.
Ethinyl estradiol: primarily hepatic via CYP3A4; norethindrone: hepatic reduction and sulfate conjugation.
Renal excretion of conjugated metabolites accounts for approximately 60-80% of an administered dose; fecal elimination via bile accounts for 20-40%.
Renal excretion of metabolites (primarily ethinyl estradiol and norethindrone conjugates) accounts for approximately 50-60% of elimination; fecal excretion accounts for 30-40%. Unchanged drug excretion is minimal (<5%).
Levonorgestrel is extensively bound to sex hormone-binding globulin (SHBG) and albumin; approximately 97-99% bound, with the free fraction being pharmacologically active.
Norethindrone: 61% bound to albumin and SHBG; ethinyl estradiol: 97-98% bound to albumin.
Apparent volume of distribution is about 1.8 L/kg, indicating extensive distribution into tissues beyond plasma water.
Norethindrone: 3.8-4.5 L/kg; ethinyl estradiol: 2.0-4.0 L/kg. Large Vd indicates extensive tissue distribution.
Oral bioavailability is approximately 100% due to minimal first-pass metabolism; levonorgestrel is completely absorbed after oral administration.
Oral: Norethindrone ~64%, ethinyl estradiol ~38-48% (due to first-pass metabolism).
No dosage adjustment required for any degree of renal impairment.
No dose adjustment required for mild to moderate renal impairment. Contraindicated in severe renal impairment or acute renal failure due to potential fluid retention and electrolyte disturbances.
Contraindicated in severe hepatic impairment (Child-Pugh class C). For mild to moderate impairment, no specific dose adjustment available; use with caution.
Contraindicated in patients with hepatic impairment, including Child-Pugh class B or C, due to impaired metabolism of estrogen and progestin. Not recommended in patients with active liver disease or history of liver tumors.
Weight ≥ 75 kg: single dose of levonorgestrel 1.5 mg orally. Weight < 75 kg: single dose of 1.5 mg is also used, but efficacy may be reduced; consider alternative methods.
Not indicated for use before menarche. For postmenarchal adolescents, same dosing as adults. Safety and efficacy established for contraception; weight-based dosing not applicable.
Not indicated for postmenopausal women; no specific dose adjustment established for elderly.
Not indicated for use after menopause due to lack of benefit and increased risks (e.g., cardiovascular, thromboembolic events). If used, monitor for fluid retention, hypertension, and glucose intolerance.
None.
Cigarette smoking increases risk of serious cardiovascular events from combined oral contraceptives. Risk increases with age and heavy smoking (≥15 cigarettes/day). Women over 35 who smoke should not use this product.
Do not use as regular contraceptive if multiple doses are taken in one cycle,Ectopic pregnancy risk reduced but not eliminated,Menstrual irregularities,Rare cases of hepatic adenoma or thromboembolic events,Use with caution in patients with severe hepatic impairment or malabsorption syndromes
Thrombotic disorders (e.g., DVT, PE, stroke, MI),Cerebrovascular disease,Hepatic neoplasia,Gallbladder disease,Hypertension,Carbohydrate and lipid effects,Ocular lesions,Hereditary angioedema,Chloasma,Menstrual irregularities,Pregnancy exclusion prior to initiation
Known or suspected pregnancy,Hypersensitivity to levonorgestrel or any component
Venous or arterial thrombotic/thromboembolic disease (current or history),Cerebrovascular disease,Coronary artery disease,Known or suspected breast cancer,Endometrial or other estrogen-dependent neoplasia,Undiagnosed abnormal genital bleeding,Cholestatic jaundice of pregnancy or jaundice with prior pill use,Hepatic adenoma or carcinoma,Known or suspected pregnancy,Hypersensitivity to any component,Smoking in women over 35
No clinically significant food interactions. Levonest releases levonorgestrel locally in the uterus, and systemic absorption is minimal. No dietary restrictions required.
No significant food interactions. Grapefruit juice may increase estrogen levels, but clinically not a concern. Avoid excessive alcohol, which may impair liver function and increase estrogen exposure. Maintain a healthy diet, as weight gain is possible.
LEVONEST (levonorgestrel) is associated with minimal teratogenic risk if inadvertently used during pregnancy. First trimester: No increased risk of major malformations based on epidemiologic data. Second/third trimester: No known adverse fetal effects from occasional use; no evidence of fetotoxicity or teratogenicity from clinical studies. However, as a progestin-only contraceptive, it is not indicated during pregnancy.
Pregnancy category X. Use of ALYACEN 1/35 (norethindrone/ethinyl estradiol) is contraindicated during pregnancy. First trimester: Increased risk of congenital anomalies, including cardiovascular defects and limb reduction defects. Second/third trimesters: Potential for urogenital abnormalities and feminization of male fetus. Exposure is associated with subsequent development of clear cell adenocarcinoma of vagina/cervix in female offspring (DES-related).
Levonorgestrel is excreted into breast milk in small amounts (estimated infant dose <0.1% of maternal dose). M/P ratio not established. No adverse effects reported in breastfed infants with short-term use. The WHO considers it compatible with breastfeeding. Caution with prolonged use may reduce milk supply.
Small amounts of contraceptive steroids and/or metabolites have been identified in breast milk. M/P ratio: Not specifically determined for this combination; ethinyl estradiol M/P ratio ~0.02-0.04. Use may reduce milk production and quality. Breastfeeding not recommended during use. Alternative contraception advised.
No dosing adjustments are applicable as LEVONEST is contraindicated in pregnancy. Pharmacokinetic changes in pregnancy do not require dose modifications because the drug should not be used during pregnancy.
Contraindicated in pregnancy; no dose adjustments applicable. Discontinue medication immediately upon pregnancy detection.
Levonest (levonorgestrel-releasing intrauterine system) provides effective contraception for up to 5 years. It can be used for emergency contraception if inserted within 5 days of unprotected intercourse. May reduce menstrual bleeding and dysmenorrhea. Expulsion risk is highest in first year, especially right after insertion. Check strings after each period. Contraindicated in acute pelvic inflammatory disease, postpartum endometritis, infected abortion, or untreated cervical infection.
ALYACEN 1/35 is a combination oral contraceptive containing ethinyl estradiol 35 mcg and norgestimate 1 mg. It is indicated for the prevention of pregnancy and for the treatment of moderate acne vulgaris in females ≥15 years of age who desire an oral contraceptive. Monitor for thromboembolic events, especially in smokers over 35 or those with migraine with aura. Use with caution in patients with liver impairment or history of cholestatic jaundice. The pill-free interval should not exceed 7 days; missed pills increase ovulation risk. Consider non-hormonal backup if vomiting or diarrhea occurs within 4 hours of dosing.
Levonest is a small T-shaped device placed in your uterus that releases a hormone to prevent pregnancy for up to 5 years.,You may have irregular bleeding or spotting for the first 3-6 months, which typically decreases over time.,Check the strings at the end of your period each month to ensure the device is in place.,Do not use Levonest if you think you might be pregnant, have a current pelvic infection, or have certain uterine abnormalities.,Most women can use Levonest safely; serious complications are rare but include perforation, expulsion, and infection.,Seek medical attention if you experience severe abdominal pain, fever, heavy bleeding, or missing strings.
Take one tablet daily at the same time each day; do not skip doses.,Use an additional non-hormonal contraceptive (e.g., condoms) if you miss a pill, have vomiting, or diarrhea.,Smoking while on this pill increases the risk of blood clots and stroke, especially if you are over 35.,Contact your healthcare provider immediately if you have chest pain, leg pain/swelling, sudden vision changes, or severe headache.,This medication does not protect against HIV or other sexually transmitted infections.,Store at room temperature, away from moisture and heat.
No interactions on record
No interactions on record
Explore head-to-head clinical comparisons of other medications in the same therapeutic classes.
Common clinical questions about LEVONEST vs ALYACEN 1/35, answered by our medical review team.
LEVONEST is a Oral Contraceptive that works by Levonorgestrel is a synthetic progestin that inhibits ovulation by suppressing luteinizing hormone (LH) surge, alters cervical mucus to impede sperm penetration, and induces endometrial changes that inhibit implantation.. ALYACEN 1/35 is a Oral Contraceptive that works by Combination hormonal contraceptive: ethinyl estradiol suppresses gonadotropin release via negative feedback on hypothalamic-pituitary axis; norethindrone induces progestational effects including cervical mucus thickening and endometrial changes, inhibiting ovulation and sperm penetration.. They differ in pharmacokinetic profiles, FDA-approved indications, and side effect profiles.
Potency comparisons between LEVONEST and ALYACEN 1/35 depend on the specific clinical indication. These are both Oral Contraceptive agents and are not directly interchangeable by dose. A physician or clinical pharmacist should guide any therapeutic switching decisions.
The standard adult dose of LEVONEST is: One tablet (levonorgestrel 1.5 mg) orally as a single dose within 72 hours of unprotected intercourse.. The standard adult dose of ALYACEN 1/35 is: One tablet (norethindrone 1 mg and ethinyl estradiol 35 mcg) orally once daily for 21 consecutive days, followed by 7 days of placebo or no tablets.. Dosing should always be individualized based on indication, renal and hepatic function, age, and other patient factors.
No direct drug-drug interaction has been formally documented between LEVONEST and ALYACEN 1/35 in current clinical databases. However, individual patient risk factors including other medications, organ function, and comorbidities should always be evaluated by a qualified healthcare provider.
The maternal-fetal safety profiles differ. LEVONEST is classified as Category C. LEVONEST (levonorgestrel) is associated with minimal teratogenic risk if inadvertently used during pregnancy. First trimester: No increased risk of major malformations based on epi. ALYACEN 1/35 is classified as Category C. Pregnancy category X. Use of ALYACEN 1/35 (norethindrone/ethinyl estradiol) is contraindicated during pregnancy. First trimester: Increased risk of congenital anomalies, including . Always consult a maternal-fetal medicine specialist before taking either drug during pregnancy or lactation.