Whatever is offered to Shiva becomes ‘Shiva-Naivedya’ and ‘Shiva-Nirmaalya’, and is believed to be not to be taken back or consumed.
Devi asked this question to Shiva. Shiva replied : O Devi! Out of my five faces, the middle one is Shyamala Ishaana and the one upward is Kalaagni-rupa. When I drank the halaahala after samudra- manthana, that poison stays always at my throat and that always provides burning sensation to my face. Whatever is offered on my face becomes not consumable by others, because those things become brahmamaya, and so one who consumes these becomes fallen.
Whatever is offered to me should be just touched by head. That has to be offered to Vishnu by Vishnu-mantra and only then it can be consumed. My nirmalya is acceptable to Vishnu only and not to the ordinary entities like deva, asura, gandharva, and manushya. (Omkarnath Rachanaavali, Sitaramdas Omkarnath,Vol 1, page 247–49)
For the Vaishnavas, Padma-Purana recommends that prasada of apShiva should not be consumed and nirmalya should thrown into well.(Ibid, page 249).
In another Purana, Shiva says that my naivedya and nirmalya are non-acceptable, but can be accepted after being touched by the Salagrama -Sila.(lbid page 249)