MICRONOR
Clinical safety rating: caution
Comprehensive clinical and safety monograph for MICRONOR (MICRONOR).
Progestin-only contraceptive; suppresses ovulation by inhibiting gonadotropin release, thickens cervical mucus, and alters endometrial lining to prevent implantation.
| Metabolism | Hepatic via CYP3A4; undergoes reduction, conjugation, and hydroxylation. |
| Excretion | Approximately 50-80% of a dose is excreted in urine as glucuronide and sulfate conjugates of norethindrone and its metabolites; about 20-40% is excreted in feces via biliary elimination. |
| Half-life | Terminal elimination half-life is approximately 5-14 hours (mean 7.6-8.7 hours). In clinical context, the short half-life requires daily dosing for contraceptive efficacy. |
| Protein binding | ~97% bound to plasma proteins, primarily albumin. Norethindrone is not significantly bound to sex hormone-binding globulin (SHBG). |
| Volume of Distribution | Apparent volume of distribution is approximately 2-4 L/kg (mean 3.5 L/kg), indicating extensive tissue distribution. |
| Bioavailability | Oral bioavailability is approximately 65% due to first-pass metabolism. Absolute bioavailability is 65-70%. |
| Onset of Action | Oral administration (0.35 mg daily): Onset of contraceptive effect occurs after 48 hours if started on day 1 of menstrual cycle; if started other times, backup contraception needed for 48 hours. |
| Duration of Action | Duration of contraceptive effect is 24 hours after each dose. Missed doses or delays >3 hours require backup contraception for 48 hours due to rapid decline in serum levels. |
0.35 mg orally once daily, taken at the same time each day.
| Dosage form | TABLET |
| Renal impairment | No dose adjustment required for renal impairment. Use with caution in severe renal impairment due to potential fluid retention. |
| Liver impairment | Contraindicated in patients with severe hepatic disease (Child-Pugh class C). For mild-to-moderate impairment (Child-Pugh A or B), use with caution; monitor for adverse effects. |
| Pediatric use | Safety and efficacy in pediatric patients have not been established; not recommended for use in females under 18 years of age. |
| Geriatric use | No specific dose adjustment recommended. Use with caution due to potential for decreased renal function and increased sensitivity to progestin effects. Monitor for fluid retention and thromboembolic events. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for MICRONOR (MICRONOR).
| Breastfeeding | Norethindrone is excreted in breast milk in small amounts (M/P ratio approximately 0.37). Clinical studies indicate no adverse effects on infant growth or development at therapeutic doses. However, caution is advised due to potential hormonal effects. The American Academy of Pediatrics considers progestin-only contraceptives compatible with breastfeeding. |
| Teratogenic Risk | FDA Pregnancy Category X. Norethindrone (MICRONOR) is contraindicated in pregnant women due to known teratogenic effects. First-trimester exposure is associated with masculinization of female genitalia (pseudohermaphroditism) and cardiovascular defects. Second- and third-trimester exposure may cause genital abnormalities in female fetuses. Use in pregnancy carries high risk of fetal harm and should be discontinued immediately if pregnancy occurs. |
■ FDA Black Box Warning
No FDA black box warning.
| Serious Effects |
["Known or suspected pregnancy","Breast cancer (current or history)","Hepatic tumors or acute liver disease","Undiagnosed abnormal genital bleeding","Hypersensitivity to norethindrone"]
| Precautions | ["Risk of ectopic pregnancy","Thrombotic disorders","Breast cancer risk (unknown)","Hepatic disease","Menstrual irregularities","Oligomenorrhea/amenorrhea","Gallbladder disease","Fluid retention","Depression"] |
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| Fetal Monitoring | Monitor pregnancy status with sensitive pregnancy tests before initiating therapy and periodically if pregnancy is suspected. In case of inadvertent use during pregnancy, assess fetal development via ultrasound to evaluate for genital abnormalities. No specific additional monitoring required for maternal health. |
| Fertility Effects | Reversible suppression of ovulation and endometrial changes. Upon discontinuation, ovulation typically resumes within 1-3 cycles. No permanent adverse effects on fertility. However, delayed return to fertility may occur in some women, particularly with long-term use. |