It seems that when a Heart Link ends, the poison-nullifying effect still remains. I had someone eat a large quantity of viper sac, and while the Heart Link was active it correctly didn't harm him at all. I then released the link with him, and the poison still didn't do anything (I gave it plenty of time). After clearing that poison and getting him a fresh dose, the new poison pulsed as it should have in the absence of Heart Link.

Thanks,
-Life Sustainer Karthor