A dose of 0.75mg of LNG in a vaginal gel was shown to prevent ovulation in a higher percentage than what has been observed with oral LNG at the approved doses for EC of 1.5mg. Results from a study with a1.5 mg LNG oral dose or a single 0 ,75mg dose demonstrated that the proportion of cycles without follicle rupture or ovulatory dysfunction within 5 days was 86% and 79% respectively (Croxatto et al, 2004) , whereas lack of follicular rupture or the presencr of ovulatory dsyfunction was 96% with administration of a LNG gel formulation. In the group of women with follicle size ≥18mm the gel induced 50% ovulation suppression while oral LNG in previous studies blocked only 16% of ovulation in the same follicle size group . These encouraging results could be explained by higher bioavailability of LNG when administered vaginally. This comparison was made using historical data from another study; therefore this study will compare vaginal administration of LNG to oral in a direct cross-over comparative study.
LNG is known to affect the cervical mucus by making it hostile to sperm penetration and its effect may appear after only a few hours following oral administration (Kesseru, 1984; Brache, unpublished observations). Application of LNG gel in the vagina close to the cervical os may exert a direct effect on the cervical mucus. The potential local action of LNG on the cervical mucus may be another important mechanism by which protection is conferred to women who do not experience ovulation suppression.
An "on demand" precoital contraceptive is an attractive alternative to the post-coital emergency contraception currently available, especially due to the higher efficacy of this route of administration on ovulation suppression as compared with oral tablets of LNG. Should the present study confirm the superiority of vaginal administration of half the dose of the oral dose of LNG approved for emergency contraception, further development of that formulation and method would be warranted.
