Your health the worst: drink! (7,2)
I believe the answer is:
bottoms up
'your health' is the definition.
(I've seen this before)
'the worst drink' is the wordplay.
'the worst' becomes 'bottom'.
'drink' becomes 'sup' (I've seen this before).
'bottom'+'sup'='BOTTOMS UP'
(Other definitions for bottoms up that I've seen before include "Down the hatch!" , "Toast" , "Cheers!" , "Your good health!" , "down it goes!" .)