What are the different progesterone-only oral birth control pills, such as Micronor (norethindrone), available for contraception?

Progestin-Only Oral Contraceptive Pills (POPs)

The currently available progestin-only oral contraceptive pills in the United States include norethindrone (Micronor, 0.35 mg) and drospirenone (4th generation progestin), with norethindrone being the most commonly prescribed POP. 1, 2

Available Formulations

• Norethindrone (0.35 mg) - First generation progestin, FDA-approved and widely available 1, 2

  • Brand names: Micronor, Nor-QD, Camila, Errin, Heather, Jencycla
  • Mechanism: Thickens cervical mucus, suppresses ovulation in ~50% of users, alters endometrium to prevent implantation 2
  • Typical-use failure rate: Approximately 6% 1

• Drospirenone (4 mg) - Fourth generation progestin with anti-mineralocorticoid properties 3

  • Newer formulation with anti-androgenic effects

Internationally Available POPs (Not in US)

• Desogestrel (0.075 mg) - Third generation progestin 4

  • More consistently inhibits ovulation compared to norethindrone
  • Similar contraceptive effectiveness to combined oral contraceptives

• Levonorgestrel (0.03 mg) - Second generation progestin 3

• Other progestins used internationally: 3

  • Chlormadinone acetate
  • Nomegestrol acetate
  • Lynestrenol
  • Ethynodiol diacetate

Mechanism of Action

POPs work through multiple mechanisms:

• Thickening cervical mucus (primary mechanism)
• Suppressing ovulation (in approximately 50% of users)
• Slowing ovum transport through fallopian tubes
• Altering endometrium to prevent implantation 2

Advantages of POPs

1. Safety in specific populations:

   • Suitable for women with contraindications to estrogen 1
   • Safe for women with hypertension 3, 1
   • Appropriate for women with history of venous thromboembolism 1
   • Can be used by women with migraine with aura 1
   • Safe during breastfeeding 1

2. Fewer cardiovascular risks compared to combined hormonal contraceptives 4

3. No estrogen-related side effects (headaches, nausea) 4

Important Administration Considerations

• Must be taken at the same time every day (within 3 hours) to maintain effectiveness 1
• No pill-free interval; taken continuously 2
• Perfect use efficacy rate: 0.5% failure rate 1
• Typical use efficacy rate: ~6% failure rate 1

Common Side Effects

• Irregular bleeding patterns - most common reason for discontinuation 4, 5

  • Breakthrough bleeding
  • Spotting
  • Amenorrhea

• Other side effects:

  • Mild drowsiness (particularly with micronized progesterone) 6
  • Fewer metabolic effects than combined hormonal contraceptives 3

Drug Interactions

• Effectiveness may be reduced by:
  • Rifampin
  • Anticonvulsants
  • St. John's wort 1

Clinical Pearls

1. Bleeding management: Adding 5 mg norethindrone acetate has been shown to significantly reduce bleeding/spotting associated with POPs 5

2. Timing is critical: Unlike combined oral contraceptives, POPs have a much narrower window for taking pills (within 3 hours of usual time) 1

3. Counseling importance: Proper counseling about expected bleeding patterns significantly improves continuation rates 4, 7

4. Starting POPs: Can be initiated at any time if reasonably certain the patient is not pregnant; backup contraception recommended if started >5 days after menses began 1

POPs represent an important contraceptive option, particularly for women who cannot or prefer not to use estrogen-containing contraceptives. The choice between available options should consider the patient's medical history, particularly focusing on cardiovascular risk factors, migraine status, and breastfeeding status.