At the very posh local farmers market

a chap was selling premium dairy products; hand churned butter, fancy milk and  ... buttermilk.  Proper buttermilk is so difficult to find that I'd forgotten about it's existence. I was sort of excited to see this rare thing but couldn't remember why.

Seeing my hesitation the dairyman handed me a leaflet with suggestions for how to use buttermilk, then he said

one of my customers drinks it but you're not supposed to do that 

Why not?

it's already been used  

!!!!!!!!!