Head-to-head clinical analysis & difference comparison: details on mechanism of action, dosing, half-life, interactions, and maternal-fetal safety.
NORETHIN 1/35E-28 vs ESTROSTEP 21
Clinician-reviewed, head-to-head comparison of mechanism, dosing, pharmacokinetics, and safety profiles.
Last clinically reviewed: July 2026 · OpiCalc Medical Review Team
Combination estrogen-progestin oral contraceptive. Ethinyl estradiol suppresses FSH and LH, preventing ovulation. Norethindrone alters cervical mucus and endometrial lining, inhibiting sperm penetration and implantation.
Estrostep 21 is a combination oral contraceptive containing ethinyl estradiol and norethindrone acetate. It works primarily by suppressing gonadotropin-releasing hormone (Gn RH) from the hypothalamus, reducing luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion from the pituitary, thereby inhibiting ovulation. Additionally, it thickens cervical mucus to impede sperm penetration and alters the endometrium to reduce implantation likelihood.
Prevention of pregnancy
Prevention of pregnancy,Treatment of moderate acne vulgaris in females >= 15 years of age who have achieved menarche, are not pregnant, and desire oral contraception
One tablet orally once daily for 28 days (21 active tablets containing norethindrone 1 mg and ethinyl estradiol 35 mcg, followed by 7 inert tablets).
One tablet orally once daily for 21 days, each tablet contains norethindrone acetate 1 mg and ethinyl estradiol 20 mcg.
Norethindrone: terminal elimination half-life approximately 8-11 hours. Ethinyl estradiol: terminal elimination half-life approximately 10-20 hours (mean ~13 hours). Clinical context: Steady-state achieved within 5 days; once-daily dosing maintains therapeutic levels.
Ethinyl estradiol: 13-27 hours (terminal); norethindrone acetate: 5-14 hours (terminal). Steady-state achieved within 3-5 days.
Hepatic: ethinyl estradiol undergoes CYP3A4-mediated hydroxylation and glucuronidation; norethindrone metabolized via reduction and conjugation.
Ethinyl estradiol is metabolized primarily via CYP3A4 and undergoes glucuronidation and sulfation. Norethindrone acetate is deacetylated to norethindrone, which is metabolized by CYP3A4 and other CYP enzymes, and undergoes reduction and glucuronidation.
Norethindrone is excreted primarily in urine as glucuronide and sulfate conjugates, with about 50-60% excreted renally; approximately 20-30% is excreted in feces via biliary elimination. Ethinyl estradiol is excreted in urine (40-60%) and feces (20-40%) after enterohepatic recirculation.
Biliary/fecal (40-50% as metabolites) and renal (30-40% as glucuronide and sulfate conjugates)
Norethindrone: ~97% bound, primarily to serum albumin and sex hormone-binding globulin (SHBG). Ethinyl estradiol: ~98% bound, primarily to albumin, with slight binding to SHBG.
Ethinyl estradiol: 97-98% bound to albumin; norethindrone: 95-97% bound to albumin and SHBG.
Norethindrone: Vd approximately 3-4 L/kg (distributes widely into body tissues). Ethinyl estradiol: Vd approximately 1.5-2.5 L/kg (moderately distributed, concentrated in reproductive tissues).
Ethinyl estradiol: 2.5-4.5 L/kg; norethindrone: 3.5-5.5 L/kg. Indicates extensive tissue distribution.
Oral: Norethindrone: ~64% (due to first-pass metabolism). Ethinyl estradiol: ~40-60% (due to first-pass metabolism and variable absorption).
Ethinyl estradiol: 38-48% (oral, first-pass metabolism); norethindrone acetate: 50-70% (oral, deacetylation to norethindrone).
No dose adjustment required for mild to moderate renal impairment. Not studied in severe renal impairment; consider alternative contraception.
No dose adjustment required for mild to moderate renal impairment. Contraindicated in severe renal impairment or acute renal failure.
Contraindicated in Child-Pugh class B or C (moderate to severe hepatic impairment). Use with caution in Child-Pugh class A; monitor liver function.
Contraindicated in acute hepatitis, severe cirrhosis, benign or malignant liver tumors, or Child-Pugh class C cirrhosis. Use with caution in Child-Pugh class A or B; consider alternative therapy.
Postmenarchal adolescents: same dosing as adults (one tablet daily for 28-day cycle). Use only after menarche; not indicated before.
Not indicated for use before menarche. Post-menarche: same dosing as adults; use lowest effective dose for shortest duration.
Not indicated for use in postmenopausal women. No specific geriatric dose; estrogen-containing contraceptives are not appropriate in this population due to increased risk of thromboembolism and cardiovascular events.
Not indicated for postmenopausal women; no established dosing in elderly. Consider alternative therapies as necessary.
Cigarette smoking increases risk of serious cardiovascular events. Women over 35 who smoke should not use combination oral contraceptives.
Cigarette smoking increases the risk of serious cardiovascular events from oral contraceptive use. This risk increases with age (especially in women > 35 years) and with the number of cigarettes smoked. Women who use oral contraceptives should be strongly advised not to smoke.
Increased risk of thromboembolic disorders, stroke, MI, especially in smokers,Hepatic neoplasia,Gallbladder disease,Hypertension,Worsening of migraine,Depression,Fluid retention,Carbohydrate/lipid effects
Increased risk of thromboembolic disorders (e.g., venous thromboembolism, stroke, myocardial infarction), especially in smokers and women over 35,Hepatic neoplasia (benign and malignant) reported,Hypertension and gallbladder disease,Carbohydrate and lipid effects; monitor in diabetic patients,Ocular lesions (e.g., retinal thrombosis); discontinue if unexplained vision loss or proptosis occurs,Hereditary angioedema exacerbation,Chloasma; avoid sun exposure,Menstrual irregularities (e.g., breakthrough bleeding, amenorrhea),Depression; discontinue if severe
Thrombophlebitis or thromboembolic disorders,Cerebrovascular or coronary artery disease,Known or suspected breast carcinoma,Estrogen-dependent neoplasia,Undiagnosed abnormal genital bleeding,Pregnancy,Known or suspected pregnancy,Hepatic adenoma or carcinoma,Active liver disease (if LFTs not returned to normal),Hypersensitivity to any component
Thrombophlebitis, thromboembolic disorders, or history thereof (e.g., deep vein thrombosis, pulmonary embolism),Cerebrovascular disease (e.g., stroke, TIA),Coronary artery disease (e.g., myocardial infarction, angina),Known or suspected breast cancer, endometrial cancer, or other estrogen-dependent neoplasia,Undiagnosed abnormal genital bleeding,Cholestatic jaundice of pregnancy or jaundice with prior oral contraceptive use,Benign or malignant liver tumors,Known or suspected pregnancy,Hypersensitivity to any component,Smoking in women over 35 years of age
No significant food interactions. Grapefruit juice may slightly increase ethinyl estradiol levels but not clinically relevant. Consistent intake recommended to maintain steady hormone levels.
No specific food interactions. Grapefruit juice may increase estrogen exposure (mild effect). Maintain consistent timing with meals to reduce nausea if needed. Avoid alcohol if liver function is compromised.
Pregnancy category X. First trimester: major congenital anomalies including limb defects, neural tube defects, and cardiovascular anomalies. Second and third trimesters: increased risk of fetal genital abnormalities (e.g., clitoral hypertrophy, labial fusion in females, ambiguous genitalia in males), and potential long-term neurodevelopmental effects.
Estrostep 21 (norethindrone acetate/ethinyl estradiol) is contraindicated in pregnancy. First trimester: increased risk of cardiovascular malformations, limb reduction defects, and neural tube defects. Second and third trimesters: associated with fetal genital abnormalities, feminization of male fetuses, and potential long-term reproductive effects. Use in early pregnancy for emergency contraception is not recommended due to potential risks.
Excreted in human breast milk; can reduce milk production and affect infant development. M/P ratio not established. Contraindicated in breastfeeding.
Estrostep 21 is excreted in breast milk in small amounts; ethinyl estradiol M/P ratio approximately 0.1; norethindrone M/P ratio ~0.5. May reduce milk production and quality, especially in early lactation. Use generally avoided during breastfeeding; consider progestin-only alternatives.
No established safe dose; contraindicated in pregnancy. Pharmacokinetic changes during pregnancy (e.g., increased clearance, volume of distribution) do not permit any therapeutic use; alternative agents recommended.
Estrostep 21 is contraindicated in pregnancy; no dose adjustments recommended as use is avoided. Pharmacokinetic changes during pregnancy (increased hepatic metabolism, volume of distribution) would require dose adjustment if used, but due to risks, alternative therapy is indicated.
Norethindrone 1 mg/ethinyl estradiol 35 mcg is a monophasic oral contraceptive. Counsel patients that breakthrough bleeding is common in first 3 cycles. If pregnancy occurs, exclude ectopic pregnancy as progestins may slow tubal motility. Monitor for hypertension and hyperkalemia in patients on spironolactone or ACE inhibitors.
ESTROSTEP 21 is a monophasic oral contraceptive containing norethindrone acetate and ethinyl estradiol. Due to its low estrogen dose (20 mcg), it may be associated with more breakthrough bleeding compared to higher-dose pills. It is indicated for contraception and may be used for acne treatment. The 21-day regimen requires a 7-day hormone-free interval. Counsel patients that smoking increases the risk of serious cardiovascular adverse effects; avoid in women over 35 who smoke.
Take one tablet daily at the same time, even if no bleeding expected.,Missed dose: if within 12 hours, take immediately; if >12 hours, take and use backup contraception for 7 days.,Common side effects include nausea, breast tenderness, breakthrough bleeding, and headaches.,Do not smoke while on this medication due to increased risk of blood clots.,Seek medical help for sudden leg pain, chest pain, vision changes, or severe headache.
Take one tablet daily at the same time each day for 21 days, then no pills for 7 days.,If you miss a dose, follow the package instructions; use backup contraception if missed more than one tablet.,Do not smoke while taking this medication; smoking increases risk of blood clots, stroke, and heart attack, especially if over 35.,Common side effects include spotting, nausea, breast tenderness, and headache; these often improve after a few cycles.,Seek emergency care if you have signs of a blood clot: sudden leg pain, chest pain, shortness of breath, or vision changes.,This medication does not protect against HIV or other sexually transmitted infections.,Take missed pills as soon as remembered; do not take more than two tablets in one day.
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 NORETHIN 1/35E-28 vs ESTROSTEP 21, answered by our medical review team.
NORETHIN 1/35E-28 is a Combined Oral Contraceptive that works by Combination estrogen-progestin oral contraceptive. Ethinyl estradiol suppresses FSH and LH, preventing ovulation. Norethindrone alters cervical mucus and endometrial lining, inhibiting sperm penetration and implantation.. ESTROSTEP 21 is a Combined Oral Contraceptive that works by Estrostep 21 is a combination oral contraceptive containing ethinyl estradiol and norethindrone acetate. It works primarily by suppressing gonadotropin-releasing hormone (Gn RH) from the hypothalamus, reducing luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion from the pituitary, thereby inhibiting ovulation. Additionally, it thickens cervical mucus to impede sperm penetration and alters the endometrium to reduce implantation likelihood.. They differ in pharmacokinetic profiles, FDA-approved indications, and side effect profiles.
Potency comparisons between NORETHIN 1/35E-28 and ESTROSTEP 21 depend on the specific clinical indication. These are both Combined 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 NORETHIN 1/35E-28 is: One tablet orally once daily for 28 days (21 active tablets containing norethindrone 1 mg and ethinyl estradiol 35 mcg, followed by 7 inert tablets).. The standard adult dose of ESTROSTEP 21 is: One tablet orally once daily for 21 days, each tablet contains norethindrone acetate 1 mg and ethinyl estradiol 20 mcg.. 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 NORETHIN 1/35E-28 and ESTROSTEP 21 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. NORETHIN 1/35E-28 is classified as Category C. Pregnancy category X. First trimester: major congenital anomalies including limb defects, neural tube defects, and cardiovascular anomalies. Second and third trimesters: increased . ESTROSTEP 21 is classified as Category C. Estrostep 21 (norethindrone acetate/ethinyl estradiol) is contraindicated in pregnancy. First trimester: increased risk of cardiovascular malformations, limb reduction defects, and. Always consult a maternal-fetal medicine specialist before taking either drug during pregnancy or lactation.