Thank You, We Have A Horse That Has Been On

Thank You, we have a horse that has been on penicillin for we thought a cold but now from listening and observing the discharge feel she has a touch of phenomena and purchased liquamycin la-200 but want to give it trans muscular injection. She weighs about 800 lbs. could you tell me how many ml.s or cc's we should administer and is the neck still a good place to inject?