We have used the menthol cough drops for 3 years.  (also grease
patties).  The menthol had to be applied while the weather was
still warm after the honey was harvested.  The bees ate the
cough drops.  Some brands of cough drops contain stronger doses
of menthol than others, so it pays to read the label.
 
Our bees succumbed to the varroa mite, however, this last winter.
 
Sue Carver
Richland, WA