What I've learned over the past five years is that you have to be very careful with this kind of mandate, or it will make people despise and doubt your whole organization. I actually think that this kind of warning label will increase the amount of cancer people get, because they'll start smoking cigarettes again, which are much worse.
Edit: To clarify, the reason people would start smoking cigarettes is not because it's an alternative to alcohol; it's because they would lose faith in health and safety warnings altogether. It's stupid, but people are stupid.