An oral contraceptive works through three actions — suppressing ovulation, changing the cervical mucus and altering the environment of the endometrium — and it has to be taken every day at a set time for that effect to continue. If a dose has been missed, what to do depends on how many hours have passed and on where you are in the cycle of the pack.
This article sets out how the contraceptive effect works, the difference between the 21-pill and 28-pill regimens and when to start them, what to do at each interval when a dose has been missed, and what to watch for if you smoke or are 35 or older.
Why does it have to be taken at the same time every day?
The core of an oral contraceptive is the suppression of ovulation. The hormones regulate the signals from the pituitary gland and block ovulation and fertilisation, which carries the greatest weight among the three actions.
The mucus becomes sticky so that sperm move with difficulty, and the endometrium too is held unfavourable to implantation, which supports the suppression of ovulation. Taking it at the same time each day is what matters.
- Suppression of ovulation — the hormonal signals from the pituitary gland are regulated so that an egg is not released from the ovary
- Change in cervical mucus — the mucus is made thick and sticky so that sperm move with difficulty
- Change in the endometrium — the lining of the uterus is kept in a state unfavourable to implantation, preventing it
Why check first whether your pack is a 21-pill or a 28-pill one?
With the 21-pill pack, 21 active pills are followed by a 7-day break, during which bleeding occurs, and a new pack is then started; with the 28-pill pack, 7 placebo pills follow on, so it is taken daily with no break.
Starting within 5 days of the beginning of a period keeps the contraceptive effect in place straight away, but starting later delays the suppression of ovulation, so additional contraception is needed for 7 days.
If a dose has been missed, what should be done at each interval?
If you are within 12 hours of the missed dose, take it straight away and carry on as usual, and no additional contraception is needed. If you are not certain, treat it as though 12 hours have passed.
If you realise after 12 hours, take the missed pill at once and use additional contraception such as a condom for 7 days. If the dose missed falls between day 15 and day 21, start the next pack straight away without the break — a break of more than 8 days can allow the suppression of ovulation to lift.
- Remembered within 12 hours — taking it as soon as you remember keeps the contraceptive effect in place with no further action
- Remembered after 12 hours — take it at once and use an additional method of contraception for the following 7 days
- Missed in the third week of the pack — skip the break and start the next pack straight away, so that no more than 8 days pass without pills
If you smoke or are 35 or older, is there anything more to watch for?
The hormones in an oral contraceptive raise blood clotting, so for smokers who are 35 or older the burden of thrombosis and cardiovascular risk is greater. Methods of contraception should be compared at a consultation before a prescription.
Long-term use has been confirmed by a 10-year study to have no effect on fertility where risk factors such as smoking or older age are absent. If you are unsure whether 12 hours have passed, or have missed several doses, having it confirmed at a consultation is safer.
