Is It Fine To Take Norethisterone Earlier Than Advised?

My periods are due 2 days after my return from holiday. As they can come up to 5 days early I have asked my doctor for norethisterone. She has advised me to take tablet at around day 18 the day before I travel. Does it do any harm to take earlier than 3 days before?
Am also on medication for heavy bleeding. Am I ok to take these if my periods do start as result of tablets not working?