Although such pills are often called "Progesterone Only Pills," they do not actually contain progesterone, but one of several chemically related compounds and there are a number of progesterone only contraceptive formulations.

Efficacy
The theoretical efficacy is similar to that of Combined Oral Contraceptive Pill (COCP). However, they are taken continuously without any breaks between packets and must be taken to a much stricter time every day (within 3 hours vs COCP's 12 hours). The real-life efficacy is therefore dependent upon user compliance; if this is a particular issue, then the 12 week action of injectable Depo Provera may be more appropriate.

POPs are not dependent upon gut bacterial flora for their absorption and so are not affected by courses of antibiotics. They will, however, be affected by any episodes of diarrhoea or vomiting.

Benefits Lacking the oestrogen of combined pills, they are not associated with increased risks of DVT, breast cancer or heart disease. With the decreased clotting risk, they are not contraindicated in the setting of sickle-cell disease. The low dose of progesterone, and absence of estrogen, make the minipill safe to use during breastfeeding; in fact, it may increase the flow of milk. Like combined pills, the minipill decreases the likelihood of pelvic inflammatory disease.

It is unclear whether POPs provide protection against endometrial cancer and ovarian cancer to the extent that COCP do.

Side effects
With no break in the dosage, bleeds do not initially occur at a predictable time. Most women tend to establish, over a few months, light spotting at approximately regular intervals. May cause mastalgia or mood swings. Weight gain is less commonly experienced than on COCP.