I HATE phenergan....
Even with a 12.5 mg dose... administered WELL beyond a minute... and diluted... I had a woman become virtually non-responsive with respirations at a maximum of 8. Because she had also been receiving morphine (not with the phenergan, but throughout the day)... the doctor requested that narcan be administered to stop those effect. Her respirations didn't increase, but she did eventually arouse enough to be in complete misery with pain.
I've known of other instances where phenergan has created similar effects in other patient's as well... with other nurses.
I won't give it IV.... IM only. The effects take a bit longer to achieve, but you don't see the respiratory depression as when you administer it IV.
I find that Zofran, administered pre or intra-op as per manufacturers guidelines... is much more effective.
Peace