I almost always use it IM as I use daily injections over LA drugs (by and large) IM is absorbed faster. It would make sence to use it SubQ as it would spread the dose over a longer period of time if you were using LA drugs or if the animal didn't obviously need the B urgently. If it needed any B urgently it would be thiamin and I'd give that seperately IM. It's pretty hard to screw up B complex injections. SubQ is slightly less likely to cause an analphalactic ( creatively spelled no doubt) reaction, or leave an abcess.