Use Q-tips to check for gapeworm. Gapeworm is rare. Have someone help you. Catch one of the most sickly chickens. One person can hold her, and hold up her head, gently raising her head up.. (straightening out her neck) the other person can pry open the beak, and use the q-tip (hold onto it well)... use the q-tip to reach down as far as you can (this will only be a couple of inches, as q-tips are short)... reach down into the back of the throat, down as far as you can and swab the throat... pull the q-tip out and examine it for gapeworms. No worms on the q-tip, no gapeworm.
Chickens can get a variety of symptoms from a variety of worms, and can die from worms. I never saw worms in my chickens poop and they did not have gapeworm, but they were sick often and I lost two, and they got dramatically better after a worming regiment with Valbazen.
If you use the BYC (this site's) search.. search for How much Valbazen, or Worming with Valbazen. If they have any type of worm this will help very quickly.
Here is a link to a website where you can see info and pictures on several kinds of worms.
How and how much, withdrawal period for eggs, etc... all tha is here, on BYC.. recent posts by dawg53 will be reliable information.
Feed stores sell antibiotics, wormers, and syringes and needles. No wormer is really made for chickens that I know of, Valbazen safely kills a variety, all types of worms.
It is given orally. 1/2 cc / ml for each standard size chicken. 1/4 cc / ml for each bantam chicken... carefully given orally, making sure not to put it into the windpipe. Given oraly back further and to the side.. avoiding the hole (windpipe) directly behind the tongue.
Valbazen is expensive and comes in a large bottle. I was able to buy from an open bottle that the guy at my feed store was willing to sell 12 ccs from... the regular bottle was $50. there.. less at Jefferslivestock, but you probably don't want to wait for another shipment, you need to act quickly.
I am not sure about giving the Valbazen when you have been giving other medications... search here for more experienced information on that.
If and when you give any wormer.... it should be given and then repeated, given again ten days later. Withhold (don't eat) eggs after giving Valbazen .... for the whole time, until the 15th day after the second worming or the 25th day from the first worming/dosage.
So far, I have noticed that worming is more often the cure than antibiotics are... and, I guess that depends on whether you have wormed for all types of worms (using Valbazen) recently or not. There are more experienced people here on BYC... seek their counsel. dawg53 is a reliable member here for worming information as well as other treatments.... search for his recent posts.
http://poultrykeeper.com/common-articles-to-all-poultry/health/worms-in-chickens-ducks-geese.html
Good luck.