My sister became pregnant while taking the Pill.

How effective is the Pill in preventing pregnancy if instructions on the packet are adhered to?


I'm sorry to hear about your sister's predicament. You don't say which Pill your sister was taking, as there are two types and their efficacy is different.

The commonest pill taken is the combined oral contraceptive pill, containing both oestrogen and a progesterone.

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If this is taken properly the failure rate is very low – about 0.2 per cent per year.

The progestogen-only pill (POP) or mini-Pill has a higher failure rate of between 2 and 8 per cent per year.

Even if the instructions are strictly adhered to, there are other reasons why failure may occur.

If a pill is forgotten, it should be taken within 12 hours (three hours for the mini-Pill), but if the interval is longer than this then it is advisable to take other precautions such as a barrier method for the next seven days.

As you can see, timing is much more critical with the progestogen-only pill than with the combined pill.

Other reasons for failure include;

  • a stomach upset such as nausea and vomiting, when the Pill may not be properly absorbed into the system
  • interaction with other drugs. Anti-epileptic drugs such as phenobarbitone and phenytoin and carbamazapine, griseofulvin (for fungal infections) and some broad spectrum antibiotics can all interfere with the efficacy of the combined contraceptive pill.

Yours sincerely

The NetDoctor Medical Team

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