Combivit is a mix of group B vitamins. Won't help if sheep are lacking selenium, copper, iodine or other minerals (for which there are boluses or drenches), but only if they have been diagnosed with vit. B12 deficiency which is cobalt dependent - or you think they might have.
There is also another disease of ruminants caused by lack of vitamin B1 which has a complex name (cerebro-cortical necrosis or CCN or PEM for polyo encephalo malacya). It means their brain cortex is going bad and they show neurological signs very much like listeriosis. Combivit IV or IM can help for these cases, or if in doubt just give it to them anyway, it's cheap and can't do any harm being only vitamins.
I also often recommend it for sheep with fluke, as can help the liver to recover if damaged by the parasites.