Certain drugs, especially antibiotics are best given at measured times so that the blood levels of the drug stay the same. If you chickens go to roost at a certain time in the evening, you can give one of the every 12 hour doses during the early morning or late at night while they are roosting. For instance, mine are going in at 6 pm, so I would get out at 6 am and give the first dose and the second at 6 pm. Or if you give the morning dose at 9 am, you can go out and give the 9 pm dose by just picking her off the roost. It is much more easy to give medicines that way when they are asleep. In most cases a medicine can be given 30 minutes on either side of the ordered time as well.