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.