What is the unstressed vowel in word poisonous?

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

Here's why:

* Stress: The stress in the word falls on the second syllable, "son", making it the stressed syllable.

* Unstressed Vowels: Unstressed vowels often have a reduced sound, sometimes sounding like the short "uh" sound. In "poisonous", the "o" in "poi" is reduced to a schwa sound (ə).