The combined contraceptive pill: how it works, risks and what to do if things go wrong

The combined pill, often just called the pill, is a type of hormonal contraception that contains oestrogen and progestogen. It prevents pregnancy mainly by stopping the ovaries releasing an egg each month.

The NHS says it is over 99% effective if used correctly all the time, and less effective if used incorrectly, for example by missing a pill. With typical use, effectiveness is about 91%. When you stop, fertility usually returns to previous levels after about a month.

There are two broad types of packet. Everyday (ED) pills are taken daily and include dummy pills, 2, 4 or 7 per packet. The 21-day pill has 21 active pills followed by a 7-day break. With both, you will usually have a bleed like a period during the dummy pills or the break. Some people choose shorter, fewer or no breaks between packs to reduce headaches and bleeding, a decision to make with a prescriber.

Take the pill at the same time each day. If you start on days 1 to 5 of your cycle, you are protected straight away. Starting later means protection after 7 days, with condoms in the meantime. The NHS lists Zoely as protecting after 7 days and Qlaira after 9 days when started after day 1.

The combined pill is not suitable for everyone. The NHS says you may not be able to take it if you live with obesity, or if you are aged 35 or over and smoke. Other reasons include:

• A history of blood clots, or a family history of clots before age 45.

• Narrowed or blocked arteries, or a stroke.

• Heart problems, heart disease or high blood pressure.

• Migraines with warning signs called aura.

• Breast cancer.

• Gallbladder or bile duct problems, liver cancer or severe cirrhosis.

• Diabetes with complications.

Common effects include breakthrough bleeding or period changes in the first few months, headaches, mood changes, nausea, dizziness and breast tenderness. A small number of people develop high blood pressure. The NHS says there is no evidence of weight gain or changes in sex drive. If side effects persist after 3 months and bother you, speak to a pharmacist or doctor.

The risk of a blood clot is very small, affecting up to 1 in 1,000 people using combined hormonal contraception. A clot could occur in a leg or lung, or cause a heart attack or stroke. Breast and cervical cancer risk are slightly increased, returning to normal 10 years after stopping, while the risk of womb, ovarian and bowel cancer is lower.

If you vomit less than 3 hours after a pill (up to 4 hours for Qlaira or Zoely), take another straight away and use condoms until you have taken 7 pills in a row without vomiting. Diarrhoea matters only if it lasts more than 24 hours; keep taking pills and use condoms for 7 days after it settles. If it happens during the last 7 active pills before a break, finish those, start the next pack without a break, skip the dummy pills and use condoms for 7 days.

What to do about missed pills depends on the pill type, how many you missed and where you are in the pack. The NHS directs people to a pharmacist, the pack leaflet or NHS 111, and says emergency contraception may be needed. The combined pill can affect how some emergency contraception works. An extra pill is unlikely to harm you but can cause nausea, vomiting or vaginal bleeding.

Beyond contraception, the pill can help with acne, heavy or painful periods, premenstrual syndrome, endometriosis and PMOS. It does not stop you catching or passing on sexually transmitted infections, so condoms are advised for that.

After a birth, the NHS gives day 21 if you are not breastfeeding and week 6 if you are, with 7 days of extra contraception. After a miscarriage or abortion, it can be started straight away within 5 days. A pharmacist may supply the pill without a prescription, and clinics and GP surgeries supply it too.