Zimecterin , says it's ivermectin 1.87% Can I use it on chickens?

Yes, you can, but it's not for on the neck. You must give that orally. I have used it by putting a dab the size of maybe two BBs smeared into the mouth of each bird. It won't work on the skin, it's an oral paste.

Do a BYC search also for "egg withdrawal time" for ivermectin- the time required for tossing eggs. It's off label use for chickens...there are many BYC threads on it you can look at.

In addition, I like jefferslivestock.com for meds and the feed store.

Some vets will do a fecal test for worms for a small fee if you choose to check the poo first.

Zimectrin Gold (horse paste) has two meds in it..     a (one) "green pea" pea size for Standard chickens and a (one) BB size (BB gun BB) for Bantams..     not more...   be careful..   Mine would not eat it on bread, they spit it out..  must taste nasty.  (Give orally, careful to dose one at a time, making sure each gets just one dose...   on bread or ?)

I have been told that chickens can build up an immunity to it rather quickly...  (so, don't use it next time you worm)....   and Ivermectin alone does not kill all worms, does not kill tapeworms.  Hence the Zimectrin Gold has two medicines and the second med in it..  (can't remember the name) is the one that kills tapeworms.
Maybe use the Zimectrin Gold first..   then, ten days later use Valbazen...     ?

I do believe it's the Ivermectin that kills mites as well, though, so that's helpful. 

Order from Jefferslivestock.com or another on-line animal supply..   watch out for freight charges for heavy items.  I have feed stores around here, three I can go to...  between the three they have everything I could need.

Withhold eggs from the first dose...     repeat dose ten days after first and then withhold the eggs for two more weeks..    safe on the fifteenth day after the second dose...   25th day from the first, from the start of treatment.... 

Good luck.

Though not a great fan of Ivermectin as it kills dung beetles etc., I have used it a couple of times when I have had birds who are a nightmare to dust or won't take oral meds.Though not licensed for chickens in Britain we can get ivermectin 'spot on' type treatment designed for racing pigeons. I estimate the weight of the bird and give a proportionate dose and then use a wash-out period of 14 days  before eating the eggs (which is the time used in sheep when oral drench is used).

Not greatly scientific, but works and has never done any known harm to my birds.

If at all possible get a vecal test done. you may have to hunt around but I am finding if I have other animals with a vet I can talk some of them into a vecal on the birds. They are a little hesitant cause they don't know how to treat. But after I reassured them I can find treatment here they ran a test. It helps to know what you are treating for as different meds take care of different pests.