I have to say that I agree with dawg53 - it is too easy to reach for the antibiotics at the first sneeze. I know from experience, because about 5 weeks after I got my 3 point of lay chickens I heard one of them sneezing a few times a day. I panicked and rushed to the vet, who gave me amoxycilin to put in the water, because she said it could be a respiratory infection that they would all get. I followed the instructions for 7 days, but it didn't change much, and as the girls all seemed ok apart from the occasional sneeze, I stopped the antibiotics. They are now almost 2 years old, and although they still sneeze from time to time, (especially when it is cold, wet and horrible weather), they have no other symptoms, and lay regularly, so they obviously weren't ill the first time round.
I started to think that maybe the antibiotics hadn't done much anyway, so I calculated the dosage that the vet had recommended in the water. It is virtually impossible to guarantee that a chicken will drink the required amount of water per day using the recommended dosage - apart from anything else, they would have to drink the waterer dry, which is almost impossible to do.
However, since then I had a chicken who had clostridium perfringens, and who was at death's door. The treatment is antibiotics (including amoxycilin, which I just happened to have to hand, as I hadn't used it all up the first time). In order to be certain that she had the full dose each day, (as specified on the packaging) I let all the other girls out in the morning, but kept the one chicken in the coop. I gave her a small portion of cooked rice onto which I had sprinkled the full day's dose of antibiotic powder (less than half a teaspoon). She loves rice, so gobbled it all up in about 5 minutes flat! I then let her out to free range with the others for the rest of the day. Result - after a couple of weeks a perfectly healthy and happy hen. (Link to the the thread
here )
In conclusion - for a simple sneeze I wouldn't immediately reach for antibiotics, but if they are essential for a more serious ailment then I would never use the water method - I would put them on a portion of food reserved exclusively for the hen you want to treat.