Phenergan is not an antihistimine.
From Wikipedia:
Promethazine (sold as Phenergan and Promethegan) is a first-generation H1 receptor antagonist antihistamine and anti-emetic/anti-nausea medication available. It can be ordered as a prescription drug in the United States, but it is over-the-counter in the United Kingdom, Switzerland, and many other countries.
Promethazine has strong anti-cholinergic and sedative as well as strong anti-allergic and anti-emetic effects. Previously in its history, promethazine has also been used as an antipsychotic, although it is generally not administered for this purpose; promethazine has only approximately 1/10 of the antipsychotic strength of chlorpromazine.
What is promethazine?
Promethazine is an antihistamine. It blocks the effects of the naturally occurring chemical histamine in your body.
Promethazine is used to treat allergic symptoms and reactions such as itching, runny nose; sneezing; itchy, watery eyes; hives; and itchy skin rashes.
Promethazine is also used to cause sedation (sleep), to assist in controlling postoperative pain, to control nausea and vomiting (especially after surgery), and to prevent motion sickness.
Promethazine may also be used for purposes other than those listed in this medication guide.
http://health.yahoo.com/drug/d00787a1#d00787a1-important