Image taken from here
"The poison-drinking lovers, lured
by poison sweet, drink more and more;
To bitterness of fatal cup,
the poison-drinkers are inured,
Though wounds are festering, and uncured,
no whispers to the vulgar goes."
Poison... it takes your death.. and if seen from a different angle, it helps you to enter in a new world.
Notice that Bhitai is referring to those people who are "attracted" to the sweet poison and are in love with it as this poison gives them strength. It makes them tougher.
One more thing is interesting here. "The poison drinkers are inured" says that this poison makes them "stronger" and on the other side, it also makes their wounds "fester". So physically (festering wounds can be seen by everyone), it is working as the normal poison works, but spiritually (may be), it has a positive impact.
If we read the next stanza "There's no bitter", we can get an idea about the source of this poison, i.e all from Beloved's side is sweet.
Sadly, i couldn't understand the last line. :( "no whispers to the vulgar goes".