The pill, or combined oral contraceptive pill, works by setting up certain conditions in your body which keep the egg and sperm away from each other. If the two don’t meet, the egg doesn’t get fertilised, and you don’t get pregnant.
There are a few different types of combined pill, they’re all over 99% effective at preventing pregnancy if you remember to take them at the same time of day, every day. Your contraceptive cover can be compromised if you don’t take the pill at the same time each day. If you’ve forgotten on the odd occasion, the pill’s effectiveness drops to 91%.
So, what is a combination pill? A combined oral contraceptive pill (COCP), has two female hormones in it: oestrogen and progesterone. Oestrogen and progesterone are naturally made in the ovaries. The pill contains synthetic (lab-made) versions. It’s called a ‘combined’ or ‘combination’ pill because it combines these two hormones.
Oestrogen and progesterone control how and when your body prepares for pregnancy, but more on how the pill stops you from getting pregnant in a moment. As mentioned earlier, there are several different types of combined pill. They’re all effective at preventing pregnancy.
The 3 main types of COCP:
Monophasic 21-day pills all have the same amount of hormone in each pill. One pill is taken each day for 21 days and then no pills for 7 days.
Phasic 21-day pills are organised into strips with 2 or 3 different coloured sections. The pills in one coloured section contain different amounts of hormones to the pills in a different coloured section. Again, one pill is taken each day for 21 days and then no pills for 7 days.
Every day pills have 28 pills in one pack. There are 21 active pills and seven dummy pills (although some can contain four dummy pills). One pill is taken each day for 28 days with no break.
There are two ways of measuring whether a pill is effective at preventing pregnancy. The first is done by ‘Perfect Use’ which is when you take the pill exactly as instructed, every day, without missing a dose. Taken perfectly, the combined contraceptive pill’s effectiveness is 99%.
‘Typical use’ is when someone takes the pill but may occasionally forget to take a dose or take it late. With typical use, the pill is 91% effective at preventing pregnancy. This means that around 9 in 100 women taking it over a year will become pregnant. One clinical study concluded that the pill is “effective, safe and well-tolerated.”
Again, in another study following 900 women in three different countries, Brazil, Egypt and China, there were only four unwanted pregnancies reported, all four because the pill wasn’t taken ‘perfectly’ (every day at around the same time). Similarly, in a study of over 2000 women taking the pill over a year, 19 became pregnant. Fifteen pregnancies of which were due to regularly missing doses or taking doses late.
So, answering, how reliable is the pill? With perfect use, very. However, missing the occasional dose or taking a dose late will affect this, increasing your odds of becoming pregnant by around 9%. The best way to ensure that the pill is as effective as possible at protecting against pregnancy is to take it at the same time every day.
Other things that can impact how effective the pill is include vomiting or diarrhoea, certain antibiotics (like penicillins and tetracyclines), the antifungal Griseofulvin, HIV medicines, anti-seizure medicines, and the supplement St. John’s Wort. Speak with your clinician if you take these, or start doing so after being prescribed the pill, and use a condom as backup protection.