What is the unstressed vowel in poisonous?

The unstressed vowel in "poisonous" is the "o" in the first syllable, "poi".

Here's why:

* Stress: The word has stress on the second syllable, "son". This means the first syllable is unstressed.

* Vowel Reduction: In unstressed syllables, vowels often reduce to a schwa sound (ə). In "poisonous", the "o" in "poi" is pronounced very close to a schwa sound.

So, the unstressed vowel in "poisonous" is the "o" in the first syllable, which is pronounced with a reduced schwa sound.