If you start using Logynon on days 1-5 of your period, you will be protected against pregnancy immediately. If you start taking it later (only if you are sure you could not be pregnant), you will need to use condoms for at least seven days until Logynon starts to work.
When taken correctly, Logynon is 99% effective in preventing pregnancy. However, if you are vomiting or have severe diarrhoea you may not be protected against pregnancy. If this happens, continue to take the pill as usual but use a condom whilst you're ill.