Postinor-2 is a form of emergency contraception (or often referred to as the "morning-after pill") and should be taken orally within 72 hours of unprotected sexual intercourse in order for it to work properly.
It consists of 2 tablets to be consumed as 1 tablet each to be taken 12 hours apart.
It works by preventing ovulation and also interferes with the transport of the egg and sperms. As it contains higher doses of hormones, it may be associated with nausea and even vomiting.
If vomiting occurs, then it would not be as effective and women will need to see their gynaecologist again for a second course. It is up to 98% effective in preventing pregnancy overall if taken within 72 hours of 1 single act of unprotected sex.
The effectiveness decreases after that and should therefore not be used as a reliable form of long term contraception.