First I've heard of poisoning someone...with immortality.

Different from a vampire bite, where the character immediately realises it and reacts.
Maribel's choice creates a more long-term, foreboding atmosphere. Renko doesn't know it yet, until the day she notices her hair doesn't grey, or she gets up after being run over by a ̶t̶r̶u̶c̶k̶ car...

Funny how this doujin ends where the story within the story ends. With us not knowing what happens to the main character. I don't think Renko would be unhappy, but Mary should have just let her known.