When you fail the reflex save you gain the entangled condition, and then are subjected to the fort save whenever you are entangled by the spell. The spell specifically says that it is a poison condition which to me sounds like the poison rules should apply, and when you fail multiple saves the dc and duration should be increased.
The spell level is 2, and lets say my ability score is 18 (+4 mod), so the initial DC will be 10+4+2=16 providing I do not have anything else going on.
After they have failed the first save, due to poison rules the save DC should now be 18, and it will increase the duration from 1d4 to 1d4*1.5 or 1d4+1d2 rounds.
But this is only if the spell benefits from multiple doses of poison rules. So does the spell get to benefit?