Some hormonal contraceptives contain only progesterone (the "minipill" or "POP"). Progesterone-only formulations may be recommended for women who are breastfeeding because they are reported to have less effect on milk production.
The efficacy of the progesterone-only pill is approximately 93%. It is important to take the pill at the same time each day, as the contraceptive effect starts to diminish after twenty hours.
Progesterone is also available as an injection administered every three months (Depo Provera) and as an implant (Implanon, Nexplanon). These options are not generally recommended for observant women because they are more likely to cause irregular spotting that cannot easily be resolved. There is also a type of intrauterine device (Mirena) that releases progesterone.
Many women experience breakthrough bleeding with progesterone only pills; this spotting may not resolve as quickly as breakthrough bleeding on combination hormonal contraceptives. Taking the pill at the same time each day may help reduce the likelihood of spotting.
Because of the risk of breakthrough bleeding, it is important to review the laws of stains (ketamim) before starting progesterone only pills (or any hormonal method). As always, one should wear colored underwear when not in niddah, avoid looking at toilet paper, and ask halachic questions rather than assuming that a stain has made one niddah. If breakthrough bleeding occurs during the shivah neki'im, one can ask a halachic question about reducing the number of bedikot. In general, a couple should be prepared for the possibility of a difficult beginning, and should be prepared to test the method for several months before abandoning it.