Assessment of a new low-dose once-a-month injectable contraceptive

Norethisterone (NET) in combination with mestranol (ME), in a macrocrystalline aqueous suspension that provides sustained release of steroids, was assessed as a once-a-month injectable contraceptive in ten healthy women of reproductive age. The ovarian function was studied before and after the intramuscular administration of 12mg NET plus 1.2mg ME, delivered as crystals of 150 micron average size. Serial blood samples were taken throughout the injection intervals in all women to measure serum progesterone (P), estradiol (E2), and NET. The NET/ME preparation effectively inhibited ovulation in 23 out of the 25 injection intervals studied. The administration of this formulation induced in some women a small degree of follicular maturation not followed by luteal activity. The endometrial bleeding patterns after each injection showed a bleeding-free period of two to three weeks. The overall data demonstrate that the parenteral administration of a macrocrystalline steroid preparation of NET/ME can bring about a sustained release contraceptive system at a substantially lower dose than those currently employed in once-a-month injectable contraception.