These pills contain only one hormone – progesterone – and are often called the mini pill. When taken correctly, the progesterone-only pill is 96–99% effective at preventing pregnancy.
How the progesterone-only pill works
It works by thickening the mucus in the cervix so that sperm cannot get into the uterus and fallopian tubes to fertilise the egg. These changes are effective 48 hours after taking a progesterone-only pill. Some types of progesterone-only pill also work by preventing ovulation.
An option if you can’t take the combined pill
The progesterone-only pill is a good option for women who can’t take the combined pill because they:
- can’t tolerate oestrogen
- have a history of blood clots (the progesterone-only pill doesn’t increase the risk of these)
- suffer from migraines
- have other medical conditions – check with your health practitioner.
The progesterone-only pill is also safe to take if you are breastfeeding.
How to take the progesterone-only pill
You can start taking the progesterone-only pill at any time in your menstrual cycle.
- If you start on the first day of your period it is effective immediately.
- If you start at any other time during the menstrual cycle, the mucus effect will provide protection 48 hours later.
To be completely safe you may want to use other contraception, such as condoms, or not have sex for 7 days after starting.
You take the progesterone-only pill every day with no breaks and no inactive pills. It is very important that you take the progesterone-only pill at or about the same time every day or it will not be as effective.
Side effects of the progesterone-only pill
Side effects are less likely on the progesterone-only pill than on the combined pill, but may include:
- increased risk of ovarian cysts
- irregular periods or no periods.
Missed progesterone-only pills
- If you forget to take a progesterone-only pill at your usual time, you will be protected as normal if you take it within 3 hours of your usual time.
- If you are 3 hours past your regular pill taking time, take it as soon as you remember and the next day go back to taking your pills at the regular time. You should use another form of contraception (eg, condoms) for 2 days to be safe.
- If you vomit within 2 hours of taking the progesterone-only pill it may not work. Take another pill as soon as you can. Diarrhoea can also stop the pill working. Use another form of contraception (eg, condoms) for 2 days to be safe.
If you have missed a pill and not used other contraception, you may want to consider the emergency contraceptive pill.
Call Healthline 0800 611 116 if you are unsure what you should do.
Stopping the progesterone-only pill
You can stop taking the progesterone-only pill at any time during a cycle, and your fertility will quickly return to normal. Your menstrual cycle will go back to how it was before you took the pill, although it might take a few months to become regular again.