Some of the so-called nectar pills are not only devoid of any merit but can also bring harm if taken, such as preventing one from attaining liberation in future lives and creating hindrances to liberation. Where do these harmful pills come from? Some are from tertons who are actually impostors and some are concocted by demons to hurt sentient beings. At times, genuine nectar pills, after being handled or made by persons who have broken samaya vows, can also be tainted. As Guru Rinpoche did not leave behind many nectar pills, accomplished masters, after retrieving them, will mix them with other nectar and medicinal herbs, then bless the pills through meditation and mantra recitations. If during this process there is one samaya violator among the attending practitioners, the pills will get tainted.
[Excerpt from Luminous Wisdom Book Series 1: The Right View ~ Liberating Living Beings]