The progestin only pill thickens cervical mucus (this blocks the sperm from meeting an egg)
The progestin only pill also disrupts the menstrual cycle, including preventing the release of eggs from the ovaries (ovulation) in 97% (of desogestrel) and 60% (other progestin) of women. 49
If used consistently (taking a pill every day without forgetting at the same time) Progestin Only Pills are more than 99% effective at preventing pregnancy for all women.
With ‘typical use’ (when women miss a pill or take it at the wrong time occasionally during the month) it is still very effective for women who are breastfeeding. There will be approximately 1 pregnancy in every 100 women using Progestin Only Pills over a year (1% failure rate). It is less effective for women who are not breastfeeding. About 3-10 women in every 100 women using progestin only pills (with ‘typical use’) over a year will get pregnant. (3-10% failure rate).46
Data suggest that all progestin only pills are equally effective at preventing pregnancy.