Valbazen, Safeguard, Zimectrin Gold... these are not "chicken wormers"... but the one chicken wormer you will be given at a feed store scares me more.. it says to not give it to egg producing chickens at all.. and, and it only kills roundworms.. Chickens get a variety of worms.. Valbazen is a good, safe, first time wormer.. it acts over several days and needs to be repeated in ten days.. also must be given to each chicken individually.. one at a time, orally.. (avoiding the windpipe/airway)...
I have a neighbor help me... we take one chicken at a time, catch them in the coop, take them out, give the medicine, then let them go -- they free range duing the day... that way, I know who I have dosed and don't miss anyone.
The best info on worming, parasites and other "issues" comes from dawg53... so.. look for his posts on worming. You can find info on how to avoid the windpipe, giving oral medications...
I can give you more info on the Valbazen -- but, you can also find the posts on it by dawg53... he has helped me greatly with treating my chickens for worms and mites (Sevin Dust for the mites).
Also, are they laying? And, have they stopped? Are they acting like they want to lay and can't? Possibly eggbound? (I would still worm the flock).
Good luck... PM me if you need to.