Oral contraceptives

How do oral contraceptives work? | Adverse effects | Factors influencing contraceptive efficacy | Other issues to note | Who should not use combined oral contraceptives? | Forgetting a pill |

How do oral contraceptives work?

Combined oral contraceptives ("the combined pill") contain both oestrogen and progestogen. Their efficacy is based on the fact that they effectively prevent egg maturation and release from the ovary. This means that the egg cannot be fertilised. These effects disappear and the situation "normalises" when the woman stops using the pill. Her first natural menstrual cycle after stopping the pill is as fertile as normal, and it is safe to become pregnant right away. A withdrawal period or a pause is thus not necessary after stopping the pill.

Adverse effects

Women using contraceptive pills may sometimes experience headache, mood changes, breast tightness and changes in weight and sexual activity. These symptoms are most common at the start of treatment and usually disappear in a few months. Nausea is rare. If it occurs, try taking the pills immediately after a meal. You can also take the pills in the evening so that the medicine (hormone) levels are highest at night. A small proportion of users experience an increase in blood pressure when using the pill and therefore cannot continue using the pill safely. The woman's blood pressure is therefore checked annually when renewing the pill prescription.

Combined oral contraceptives are associated with a slightly increased risk of deep vein thrombosis (blood clot). However, this risk is quite small in young, healthy, non-smoking women of normal weight. The risk of arterial blood clots is even smaller.

Several different contraceptive pill options are available; they differ from each other in terms of the oestrogen content in the pill and the oestrogen and progestogen molecules used. A woman may react differently to different contraceptive pills, and it is therefore a good idea to try a different product if required. Pauses in the use of contraceptive pills, patches and vaginal rings are not recommended since the small associated risk of blood clot formation has been found to be highest during the first three months of use, even if the woman is restarting the treatment after a temporary pause.

Breakthrough bleeding and spotting:

Sometimes, particularly during the first months of pill use, you may experience bleeding (breakthrough bleeding or spotting) even if you use the pills regularly. Minor bleeding is nothing to worry about; you can keep taking the pills as usual. However, you should contact a doctor if the bleeding is painful or as heavy as menstruation or if it persists for more than a week.

Missing periods during oral contraceptive use:

Menstrual bleeding often becomes lighter and may stop completely. When you miss your period for the first time, it is a good idea to take a pregnancy test. Once pregnancy has been excluded, you can keep on taking the pills as normal after the normal 7-day break. Get in touch with your doctor if you have not had a period for 4 months.

Factors influencing contraceptive efficacy

The efficacy of oral contraceptives may be impaired by certain medicines, particularly certain antiepileptics and some antibiotics, and also by activated charcoal tablets. Some natural and herbal products, including those containing St. John's wort extract, may significantly impair the efficacy of oral contraceptives. For the majority of natural and herbal products, there is unfortunately no research evidence on how they interact with medicines.

If you are prescribed antibiotics, remember to tell the doctor you are on the pill. The majority of antibiotics can be used together with the pill without the risk of impairing the contraceptive effect. However, if the antibiotics cause distinct watery diarrhoea or vomiting, the pill may not be fully absorbed from the gastrointestinal tract and the contraceptive effect may be at risk; see next paragraph.

Potentially impaired efficacy in connection with vomiting and diarrhoea:

If you vomit or have diarrhoea within 4 hours of taking the pill, it is possible that the hormones have not yet been absorbed in your body. Continue taking the pill as normal but also use additional contraception (a condom) until your menstrual periods start.

Other issues to note

Women using oral contraceptives (the pill) have lower rates of uterine fibroids, endometriosis and ovarian cysts and a lower risk of pelvic inflammatory disease compared to non-users.

Attempts have been made to assess the effects of oral contraceptive use on the risk of breast cancer, particularly since long-term hormone therapy in the menopause is known to increase the risk of breast cancer. Based on currently available information, oral contraceptives do not involve a clearly established increase in breast cancer risk, but the use of combined oral contraceptives is not recommended for women with a history of breast cancer.

Oral contraceptives do not impair the woman's future fertility, but higher age does. Women who use contraception for a longer time - no matter which method they use - and only start trying for a family well after the age of 30 will have more problems getting pregnant as female fertility decreases with age.

Forgetting a combined contraceptive pill

If less than 12 hours have passed since missing a pill, the contraceptive effect of the product has not been impaired. The missed pill should be taken as soon as it is remembered and the following pills at the usual time.

If more than 12 hours have passed since missing a pill, the contraceptive effect of the product may be impaired. 

The instructions below apply you have forgotten one pill. If you have forgotten several pills, it is a good idea to get in touch with the FSHS public health nurse or the local health centre.

First week of pill use

Take the missed pill as soon as you remember it, even if this means taking two pills at the same time. The following pills should be taken at the usual time. You should also use a condom for the next 7 to 9 days (depending on the oral contraceptive brand you are using). Women who had intercourse during the 7 days before missing a pill might become pregnant. The risk of pregnancy increases the more pills were missed and if the pills were missed soon before or soon after the regular pill-free break.

Second week of pill use

Take the missed pill as soon as you remember it, even if this means taking two pills at the same time. The following pills should be taken at the usual time. If the pills have been taken regularly according to instructions for 7 days before the first missed pill, additional contraception is not required. Otherwise, you should also use a condom for the next 7 to 9 days (depending on the oral contraceptive brand you are using).

Third week of pill use

If you miss a pill, you can choose between two different options:

  • 1. Take the missed pill as soon as you remember it, even if this means taking two pills at the same time. The following pills should be taken at the usual time. Start the next blister pack as soon as your current blister pack runs out, i.e. without a pill-free break. Withdrawal bleeding will probably not occur before the new pack runs out, but you may have spotting or breakthrough bleeding during pill use.
  • 2. You can also stop using pills from your previous blister pack and start the 7-day pill-free break. Check the number of days on which you have missed your pill and count these in the pill-free break. Start taking pills from a new blister pack 7 days after the last pill.

If several pills have been missed and you have no withdrawal bleeding during the first normal pill-free break, you may be pregnant.

Who should not use combined oral contraceptives (the combined pill)?

Oral contraceptives are not a good choice for smokers and in some cases should not be used at all. Smokers over 35 should not use combined oral contraceptives at all since smoking greatly increases the risk of serious cardiovascular adverse effects, i.e. risks related to the heart and blood vessels.

Oral contraceptives are not recommended if the woman is severely obese (BMI >35 kg/m2).

Combined oral contraceptives are currently not recommended for women who have migraine with aura. Women under 35 who have migraine without aura can use combined oral contraceptives if they are otherwise healthy.

In addition to the above, there are some underlying diseases and cases where oral contraceptives are not recommended. These include a history of venous or arterial thrombosis (blood clot in a vein or artery), hypertension, diabetes with vascular complications or an active liver disease.

The pill is not suitable for everybody, and a decision on whether it can be used is usually made during an appointment with a doctor.

Information sources:
Terveysportti-Duodecim [in Finnish]

This article was written by
Johanna Castrén, GP; reviewed by SI and SV on 21 Jan. 2013 


Key words: Oral contraceptives, Contraception