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.
I've never heard it to be an antihistimine. All I've ever known til now is for anti-nausea..