How does the combined pill prevent pregnancy? Uses and missed-pill rules

How does the combined pill prevent pregnancy?

The combined contraceptive pill contains oestrogen and progestogen. It prevents ovulation and is taken according to the instructions for the particular product. It does not protect against sexually transmitted infections.

The NHS combined-pill guide describes effectiveness as over 99% with consistently correct use and approximately 91% when not used correctly, measured over a year. These population estimates are not a guarantee for an individual.

Not every pack follows a 21/7 pattern

NHS guidance identifies separate starting instructions for products such as Zoely and Qlaira. Do not assume that every everyday pack contains 21 active and seven inactive tablets.

If a pill is missed

Advice depends on the product, whether the tablet was active, how many were missed, where you are in the pack and whether unprotected sex occurred.

The Devon NHS sexual-health guide shows why timing matters:

• Missing active pills around the beginning of a pack or extending a break can require assessment for emergency contraception.
• Missing pills near the end of the active-tablet sequence can change whether a scheduled break should be taken.
• Other situations have different requirements for additional protection.

Check your leaflet and contact a pharmacist or sexual-health clinician promptly with the pack name and dates. If unprotected sex occurred, do not delay advice while trying to interpret a generic rule: emergency contraception is time-sensitive. Vomiting, severe diarrhoea and interacting medicines can also affect protection.

This guide intentionally does not provide one missed-pill instruction for all formulations.

Suitability and monitoring

The NHS eligibility guidance identifies important factors including smoking at age 35 or over, clot or stroke history, migraine with aura, blood-pressure problems and certain cancers or liver conditions.

A clinician should apply the relevant eligibility criteria. A contraindication must not be softened by a blanket statement that “many are not absolute”. Review medicines, blood pressure and changing health circumstances, and discuss alternatives if oestrogen is unsuitable.

Preparing for assessment

Bring the exact pack, recent missed-tablet dates, other medicines and relevant health history. For a different menstrual question, see norethisterone and period delay; period-delay treatment is not interchangeable with contraception.

Prescrivia is an intermediary. Independent clinicians assess suitability; no prescription, access or contraceptive outcome is guaranteed.

General information only, not medical advice. Follow the leaflet for your actual product and discuss decisions with a qualified clinician. See the current clinical, legal and language review statuses for this edition.

Sources

• NHS guidance: pack differences, tailored regimens, brand-specific starting rules, effectiveness and interactions.
• NHS sexual-health service guidance on missed pills, first-week emergency-contraception considerations and end-of-pack break changes.
• NHS guidance on clinical suitability including smoking at 35+, clot history, migraine with aura and other risk factors.

Sources checked online on 6 October 2026. Country-specific guidance does not establish availability or prescribing rules throughout Europe.