If you take it correctly, very. Over 99%. So to put this into perspective, out of 100 women taking Norgeston over a 12-month period, less than 1 will get pregnant. This is known as ‘perfect use’ of the pill.
It’s not quite as effective if you miss pills here and there, or are late in taking it. Around 91%. So out of 100 women taking it for a year, 9 will become pregnant. This is called ‘typical use’ of the pill.
So if you want to get as much protection as you can from it, aim for ‘perfect use’.