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Home / Contraception / Contraceptive effectiveness - The 4 Reasons Contraception Fails

Contraceptive effectiveness - The 4 Reasons Contraception Fails

Contraception is important. It is something you want to know you can rely on. Most hormonal contraceptives work over 98% of the time, and thankfully, hormonal contraceptive failure is a relatively rare occurrence. However, all methods can, and do, fail. Even with perfect use, a copper IUD fails 0.6% of the time, compared to spermicides that fail 18% of the time. Typical use failure rates are even higher.

Different methods work better for different couples and are influenced by factors like frequency of sex, side effects of medications and personal preference. Whatever method you use, always follow the advice of your doctor, otherwise, you risk its effectiveness.

Read on to learn about the top 4 reasons contraceptive methods fail and what you can do to prevent that.

1. User error

By far, the most common reason for contraceptive failure is user error. Even the most effective method will fail to work if instructions are not properly followed. Not taking a pill or not applying a patch correctly will lessen its effects. Depending on the product, your body needs a steady dose of estrogen and progestogen for effects to last throughout the month. Not having those hormones available will mean your body stops the processes that prevent pregnancy. You will begin to ovulate, the lining of your uterus will thicken making it easier for sperm to embed, and the mucus that covers your cervix will thin.

Missing a single dose does not guarantee you will become pregnant, though it does drastically increase the risk. Missing more than one pill is a bigger problem, and you should assume that you have lost contraceptive effects. If you are taking oral hormonal contraceptives, such as the combined pill or the mini pill, it is very easy to forget to take your daily pill; this is why many women like to set an alarm.

The best way to make sure you are taking your contraceptive correctly is to read the patient information leaflet. The patient information leaflet contains everything you need to know about a medication, including ingredients, side effects and how to use it. When in doubt, contact your doctor or women’s health professional (gynecologists).

2. Medication interactions


Antibiotics are a type of medicine prescribed to treat bacterial infections. They help your immune system to clear infections by interfering with bacterial cell functions. Unfortunately, they can also interfere with the hormonal contraceptives you use.

In particular, two enzyme-inducing antibiotics, rifampicin and rifabutin, have been identified as influencing the efficacy of contraceptives. These are generally prescribed to treat tuberculosis (TB) and meningitis.

These drugs increase the production of enzymes in your body that then break down hormones. As oestrogen and progesterone are hormones, and the essential components of contraceptives, the danger is that these antibiotics will stop the pill from having any effect.

In fact, a report conducted by the Medicines and Healthcare products Regulatory Agency (MHRA) found that the incidence of unintended pregnancies was thirteen times higher for women taking rifampicin and rifabutin.

Though these are the usual suspects in the antibiotic class, it does not mean other medications will not affect the way your contraceptives work. For that reason, you should always inform your doctor of any antibiotics you are taking when being prescribed birth control.

Other medicines

If you are epileptic and are taking anti-epilepsy drugs (AEDs), you should ask your doctor about how they might affect hormonal contraceptives. Many AEDs, though not all, are enzyme-inducing.

The same can be said for certain anti-HIV medications. All women living with HIV can use all forms of contraception, though they should be aware of an increased risk of pregnancy if not using a barrier method alongside hormonal methods.

It is important to tell your doctor about all the medicines you are taking, even if they are home remedies. One example is St. John’s Wort. St. John’s Wort is a popular herbal treatment, commonly used for mental health issues like depression and insomnia. Unfortunately, it also increases how quickly oestrogen is metabolised.

Image of different medications

3. Stopping breastfeeding

If you are a recent mother and are still breastfeeding your child, you will find you aren’t having any periods. When you breastfeed, your body produces more of a hormone called prolactin and less of a hormone called gonadotropin. In response, your ovaries will stop releasing eggs, and no eggs means no pregnancy. However, in certain cases, your body may still produce eggs. As with all contraceptive methods, there is a failure rate. Speak to your doctor for more details.

If you decide to have sex during this time, you will not necessarily need to use any contraceptives, although it is recommended, as you will be using the lactational amenorrhoea method (LAM). Although for the sake of being safe and to protect you from sexually transmitted diseases (STDs), you may still want to avoid unprotected sex and use barrier contraception (e.g. male condoms/ female condoms).

The risk of LAM comes when you stop breastfeeding; contraceptive effects only last for as long as you’re producing prolactin and gonadotropin. At the six month mark, or whenever you introduce solid foods to your child’s diet, you should have a conversation with your doctor about birth control as your menstrual cycle will restart.

Mother breastfeeding a child

4. Vomiting and diarrhoea

The intrauterine device (IUD), the patch, the vaginal ring, the contraceptive implant and other non-oral contraceptives are unaffected by vomiting and diarrhoea. Oral methods are a different story altogether.

For oral medication to work, it has to be digested before passing into your bloodstream. The process takes time, not a lot, but enough that an upset stomach can impact its efficacy. If you throw up or suffer from diarrhoea within two hours of taking birth control pills (mini pill and combined) you should treat the dose as missed.

If you vomit or experience diarrhoea over the course of 24 hours, you should count every day with sickness as a missed dose. You should continue to take your pills as normal, however, most likely you will not be protected. You should consult the information in the patient leaflet or contact your doctor for instructions regarding your specific type of oral contraceptive pill.

You should also use a barrier contraceptive (condom) for seven days after sickness. The reason is that it takes two days for the mini pill to increase the consistency of cervical mucus but seven days to completely stop the process of ovulation.

You should also be aware that spells of vomiting and diarrhea can impact how emergency contraception works. If you have any uncertainty surrounding your contraceptive use, it is sensible to consider pregnancy tests until you have complete confidence in your chosen method.

Woman vomiting into sink

For more information about reproductive health, different methods of contraception, and sexually transmitted infections (STIs), you can visit euroClinix’s different condition and product pages; simply navigate through them with the menu bar at the top of this page.

Medically reviewed by Dr. Plauto Filho Written by our editorial team Last reviewed 12-11-2023

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