What is the interaction between doxycycline and Lo Loestrin (ethinyl estradiol and norgestimate)?

Interaction Between Doxycycline and Lo Loestrin (Ethinyl Estradiol and Norgestimate)

Doxycycline does not significantly reduce the effectiveness of Lo Loestrin, but backup contraception is recommended during concurrent use due to theoretical concerns about reduced hormonal contraceptive efficacy.

Mechanism of Interaction

Doxycycline has historically been thought to potentially reduce the effectiveness of hormonal contraceptives through:

1. Alteration of gut flora that metabolize estrogens
2. Increased hepatic metabolism of contraceptive steroids
3. Reduction in enterohepatic circulation of estrogens

Evidence on Interaction

The evidence regarding this interaction is mixed:

• Research studies show no significant pharmacokinetic interaction between doxycycline and hormonal contraceptives:

  • A controlled clinical trial found no statistically significant differences in serum levels of ethinyl estradiol or norethindrone when doxycycline (100 mg twice daily) was administered 1
  • Another study demonstrated the absence of pharmacokinetic interactions between etonogestrel/ethinylestradiol released from vaginal rings and doxycycline 2

• However, the FDA labeling for doxycycline notes: "Concurrent use of tetracycline may render oral contraceptives less effective" 3

Clinical Management

Despite limited evidence of a significant interaction, a cautious approach is recommended:

1. Primary recommendation: Use backup contraception (e.g., condoms) during doxycycline treatment and for 7 days after completing the antibiotic course 4

2. Monitoring: Be alert for signs of breakthrough bleeding, which could indicate reduced contraceptive effectiveness

3. Administration considerations:

   • Take doxycycline with a full glass of water
   • Separate doxycycline doses by at least 2 hours from dairy products, antacids, and supplements containing calcium, iron, magnesium, or sodium bicarbonate 5
   • Take doxycycline with food to reduce gastrointestinal side effects 5

Special Considerations

• Extended cycle regimens: If using an extended cycle regimen of Lo Loestrin, eliminating the hormone-free interval will minimize fluctuations in medications that interact with combined oral contraceptives 4

• Missed pills: If pills are missed during concurrent doxycycline use, follow standard missed pill protocols but consider emergency contraception if unprotected intercourse occurred 4

• Duration of doxycycline: If doxycycline is prescribed for longer than 3 weeks, consider discussing alternative contraceptive methods that are not affected by antibiotics (e.g., IUDs, implants) 5

Common Pitfalls to Avoid

1. Assuming no interaction: Despite limited evidence of significant pharmacokinetic interactions, clinical practice guidelines still recommend backup contraception

2. Neglecting patient education: Patients should be informed about:

   • The need for backup contraception
   • Proper timing of doxycycline administration relative to food and supplements
   • Signs of possible contraceptive failure (breakthrough bleeding)

3. Overlooking the importance of adherence: Emphasize the importance of taking both medications as prescribed, especially since doxycycline's side effects (nausea, vomiting) might interfere with oral contraceptive adherence

By following these recommendations, patients can minimize the potential risk of contraceptive failure while using doxycycline and Lo Loestrin concurrently.