in Lucretius honey is put onto the cup of wormwood (which would be to emphasize a pleasant way of speaking).
It seems to me that Lucretius is using the honey/wormwood as a metaphor for administering medicine to a sick patient. Frank criticism is often compared to medicine as well. This is in the commentary to Philodemus' work:
Not only is frank speech "akin to friendship," it is the "language of friendship" and the "most potent medicine in friendship," to be employed in mutual moral reform among friends.
Philodemus uses a pleasant method of delivery (poetry) to administer what some see as bitter medicine (no afterlife, no gods looking over us, etc.). Once administered, however, the philosophy does the person good, in fact cures them of fear and anxiety etc. Frank criticism is another tool to administer medicine.