The combined contraceptive pill, commonly known as ’the pill’, is still the most common form of contraception for women in the UK, over 50 years after its invention. Containing a combination of the synthetic female hormones oestrogen and progesterone, it works by preventing ovulation and making it more difficult for sperm to reach the ovaries (by thickening the cervical mucus) as well as thinning the lining of the womb, meaning a fertilised egg is less likely to attach.

It’s known to be 99% effective in preventing pregnancies when used as directed. Good statistics, but it’s important to be aware that there are things that can alter its efficacy that you might never have considered. So take note…

Be careful where you store your pill packs. While you want to keep them in a safe place, the windowsill in summer – or winter - isn’t a great idea.

“Recommended storage for the pill is 25ºC or below, away from light,” advises Dr Zoe Watson, a GP and founder of wellbeing platform Wellgood Wellbeing. “Like pretty much all medications, extreme heat, moisture and light can damage them and reduce their efficacy. It’s advised not to expose your pill to temperatures below 15ºC or above 30ºC for prolonged periods of time.”

The bathroom cabinet might be a better bet. 

Some studies have suggested that certain bowel conditions can affect the absorption of the pill, and if you’re experiencing persistent diarrhoea it’s advisable to use extra contraceptive precautions.

“If someone was having a flare-up of Crohn’s, ulcerative colitis or indeed irritable bowel syndrome, which resulted in them having persistent diarrhoea for longer than 24 hours then yes, this could affect the efficacy of the contraceptive pill,” advises Dr Watson. “However, if their disease is well controlled and they are stable and symptom-free, then it shouldn’t have an impact.”

If you’re in any doubt, consult your GP about your concerns. 

The general advice is that you should try to take your pill at the same time every day, or at least within a three-hour window. Otherwise, your chances of getting pregnant can increase – but this is complicated, as the chances will depend on whether you’ve missed a pill completely and how many you’ve missed. 

“This only matters if you forget to take it completely,” advises Dr Watson, “and even then, you do have a little bit of a grace period.” However, the jury is out on the timing of taking your pill, so to be safe, go with the three-hour window advice. 

Dr Watson says it’s important to note that “the advice can vary slightly between pill preparations, so always thoroughly read the info leaflet for your particular pill, but in general, missing one pill anywhere in a pack isn’t a problem – you’ll still be protected from pregnancy.

“Missing two or more pills or starting a new pack late might affect your contraceptive cover, so extra contraceptive precautions will need to be taken.”

“There are certain medications which can alter the efficacy of the pill because they speed up the rate at which the hormones within the pill are metabolised by your liver,” explains Dr Watson. “This group of drugs are known as hepatic enzyme-inducing drugs and include rifampicin-like antibiotics, some epilepsy medications, some antiretroviral drugs used to treat HIV and alternative therapies such as St John’s wort.”

In addition, if any medications or herbal supplements cause diarrhoea or vomiting it would reduce your contraceptive protection, so always be aware of this and take extra precautions. While it’s not always possible to anticipate this in advance, some supplements, such as iron tablets, are known to cause nausea and diarrhoea, but you can reduce the risk of this by taking them with food.

Most of us are aware that sickness and diarrhoea will affect our contraceptive protection, but for the sake of completeness, we couldn’t not mention this one – especially as Dr Watson advises that along with missing a pill, vomiting or diarrhoea is the biggest reason for pill failure.

“If you vomit within three hours of taking a combined pill or within two hours of taking a progestogen-only pill, it probably will not have been absorbed by your body, so you should take another pill straight away,” she warns.

“As long as you’re not sick again, you’re still protected against pregnancy. But if you continue to be sick or have diarrhoea for more than 24 hours, this can mean your protection against pregnancy is affected, therefore extra contraceptive precautions need to be taken – for example, condoms.”

So if your hangover is making you as sick as a dog, make sure that you’re covered, contraception-wise. 

While other factors may have a fairly low impact on pill efficacy, it’s certainly better to be armed with all the facts – better safe than sorry, as your mum always says.

For more information, you can check out the Faculty of Sexual and Reproductive Healthcare guidance on different forms of contraception. 

Images: Getty