excerpt from:
"The Medusa and the Snail: More Notes of a Biology Watcher."
by Lewis Thomas. ISBN 0-553-25913-X   Bantam Books.

pg 48 "On Etymons and Hybrids"
...
The word poison came by a devious route, like a long-delayed afterthought.
It derives from "poi", to drink, becoming "potare" in Latin, whence "potion"
(and also "symposium," from "syn", (together/with) plus "posis", to drink).
The venomous meaning did not come until the notion of love potions evolved,
and the the idea of poison came to consciousness.

There is the same strange history behind the word "venom". This began as the
simple word, "wen," meaning to wish or will, leading more or less directly
to "win". Along the way, a fork led to "venus," "venery," and "venerate,"
all indicating varieties of love. The love potion was called "venin," and
somehow this gradually acquired today's sense of venom.

Nobody can explain why "poison" and "venom" came from love potions. Perhaps
it was because the pharmacology of the day was primitive and chancy, a very
thin line away from toxicology. Or maybe there was a commonsense consensus
that any sort of chemical additive intended to induce false love is, by
its very nature, a fundamental poison. It tells something important about
the good taste of earlier human beings that venom and poison were taken out
of the hands of artificial lovers and transferred to the stings of insects
and the fangs of serpents.
...