Used correctly, Noriday can be up to 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. If this continues for more than 24 hours you should follow the same steps as a missed pill.
If you are more than 3hrs late taking it, you will not have contraceptive protection.