Michie, see:
https://www.backyardchickens.com/forum/viewtopic.php?id=420520 Below is just a brief summary of that post. Hopefully, Jim (dawg53) will see this post and advise you whether or not you should bother with doing Wazine 17 at all since your chickens are only four months old.
I don't know, but if your chickens have been in your yard or free ranging, I'd bet money that they do have worms, but maybe not a possible plethora of roundworms that would necessitate a one-time treatment with Wazine 17, i.e., you might just want to get yourself a one ml ORAL syringe and give them each 1/2 ml of Valbazen and dump your eggs for three weeks.
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Usually you can't be 100% sure whether or not your chickens have worms. I didn't think that I had a problem until I took my rooster to vet's where he stayed for a week being treated for respiratory infection and fungus. Fecal samples showed round/other worms, much to my chagrin. I was going to treated them all immediately, but the vet had me to treat them all first for preventing any possible spread of respiratory disease, i.e., I'm just now getting to where I can treat them for all the worms they have that I TOO didn't think they have.
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WORMING YOUR CHICKENS - FROM THE GOSPEL of JAMES (Chapter dawg53 1-2-3)
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1. Wazine17 - two ounces in two gallons of water for one day - dump eggs for two weeks
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...Ten days later:
2. Valbazen -one half cc/ml for standard size chickens, one quarter cc/ml for smaller chickens including silkies. You can use a syringe to squirt it down their throats individually or you can inject it into a small piece of bread and give each chicken a piece of bread....they gobble it up. - dump the eggs for two weeks
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3. ...after a couple of days of using both wormers....give your chickens plain yogurt or buttermilk(probiotics), canned beef catfood (extra protein), all mixed in their feed and give it to them to build up their immune systems, do this about 3 days in a row. Then you'll have healthy, happy chickens lol.
The next time you worm,say in about 6 months or whenever you see fit...you can use the valbazen first, no need to use the wazine unless you want to. Please PM anytime and I'll be happy to help you with worming. Jim.