If I run out of ivermectin because I have to order it will it be ok just to carry on when I get it?

... or should i start the whole process again as it say one drop on the back of the neck for three weeks i am on the last week and have run out. How many times a year do i use this product one a year? Every 6 months? And can my canarys bath during the treatment course as i thought letting them have a bath would wash the ivermectin off should i wait a week to let the ivermectin work many thanks