As the label states it is for cattle, pigs and horses, for a single dose only, this is all that the manufacturers have tested and therefore guarantee safety and efficacy on that basis.
As vets, we can use medicines in a different way than the label states (eg. In this case either for use in sheep and for repeated injections) under the cascade system which relies on our knowledge of pharmaceutical properties and manufacturers advice.
As such, and because there is no painkiller licensed in sheep, I frequently give sheep a single dose of metacam at the dose of 1ml per 40kg (2.5ml per 100kg) and haved also repeated the injection at 2-4 days as required in sheep and cattle.
Every time we use metacam (or generic version) in any species we run up the benefits of the pain relief and reduction of inflammation against the risks of it affecting the organs (kidneys and liver can get damaged by non steroidal anti inflammatory drugs including metacam, and there is a slight risk of stomach ulcers, though this is far more likely in small animals). Every time a vet decides to give or not give a medicine, this process goes on to decide when, if or how much drug to give.
Any time a vet gives a drug they should be happy to discuss why they are giving it and you are always within your rights to ask and discuss any treatment given to your animals.
Hope this helps a bit