Two families, two mechanisms
| Combined pill | Progestogen-only pill | |
|---|---|---|
| Contents | An oestrogen + a progestogen | A progestogen alone |
| Main mechanism | Blocks ovulation by suppressing the hormonal surge that triggers it | Thickens cervical mucus; blocks ovulation depending on the molecule |
| Regimen | 21 days then 7 days off or inactive tablets, or continuously | Every day without a break, at a fixed time |
| Missed-pill margin | About 12 hours | 3 or 12 hours depending on the molecule — check the leaflet |
The bleeding during the break on a combined pill is not a true period but a withdrawal bleed, caused by the hormonal drop. It has no physiological function: taking the pill continuously, where the product allows, is an accepted practice.
Effectiveness
| Pregnancies per 100 women per year | |
|---|---|
| Perfect use | under 1 (about 0.3) |
| Typical use | around 7 |
That gap from one to seven is almost entirely down to missed pills and interruptions. It is the central argument for methods that depend on no daily action — implant, IUD — for anyone who knows their own consistency is fragile.
If you miss a pill
The exact rules are in the leaflet and vary by molecule. The general principle, for a combined pill:
- One pill missed, caught within 12 hours: take it at once, carry on as normal. No extra protection needed.
- More than 12 hours, or two or more pills: take the most recent missed pill, carry on, and add condoms for 7 days (WHO/JHU handbook).
- If the miss falls in the last week of the pack: run straight into the next pack with no break.
- If there was intercourse in the days before the miss: consider emergency contraception.
- Vomiting within 3 hours of taking it (4 hours for some pills) (NHS, 2024), or diarrhoea lasting more than 24 hours: treat the pill as not absorbed and apply the missed-pill rule.
Contraindications and precautions
These apply above all to combined pills, where the thrombo-embolic risk is the main concern. They include a personal history of deep-vein thrombosis or pulmonary embolism, certain clotting disorders, migraine with aura, uncontrolled hypertension, smoking after 35, some cancers, and a period of prolonged immobility. Progestogen-only pills can be used in several of these situations, which is one of the common reasons for prescribing them.
Interactions: enzyme inducers — some anti-epileptics, rifampicin, several antiretrovirals, St John's wort taken over the counter — can reduce the pill's effectiveness. Any new prescription, and any supplement bought without one, is worth mentioning.
Signs that mean seeking help immediately
Pain or swelling in one calf, chest pain, sudden breathlessness, an unusual and severe headache, sudden visual disturbance, weakness on one side of the body. These warrant emergency assessment.
Return of fertility
It is quick: pregnancy is possible from the next cycle, and fertility usually returns to its previous level within about a month (NHS, 2024). The pill does not "tire" the ovaries and needs no break. The first cycles may be irregular while your own rhythm re-establishes itself.
