Thanks for reaching out to EmpowHER with your question!
I took a look at Postinor's website to read their directions. What kind of postinor are you taking? Is it the kind with one tablet or two? According to their directions, you must take Postinor within 72 hours of unprotected sex.
Of course, the sooner you take it the better. If you took it at 18 hours, that's good because it is early on in the 72 hours. I don't think you need to take the 2nd pill unless you purchased a 2-pack type of Postinor.
With the 2 pack, you take the 2nd pill exactly 12 hours after the 1st pill.
When taken correctly, emergency contraception is very effective in preventing pregnancy. If the pack you bought is a 2-pack, make sure you do take the 2nd one 12 hours later.
I hope this helps!