Promethazine is a very sedating anti-histamine thus it boosts the nod associated with all opiates as it adds to the central nervous system depression. Alot of sedating anti-histamines such as diphenhydramine, dimenhydrinate, doxylamine, hydroxyzine and of course promethazine are used by opiate users to add to the nod of opiates while helping to stop some of the side effects such as histamine release and nausea. Doctors use combinations of opiates and anti-histamines such as Codeine/Promethazine to decrease side effects and increase the therapeutic effects while drug users use this combo in a effort to increase the nod and euphoria. I do find that anti-histamines such as hydroxyzine and promethazine can increase the euphoria of strong opiates abit but mostly they just add to the sedation.
I have heard some people say that Promethazine increases the metabolism of codeine into morphine via inducing enzyme cyp2d6 but i have not seen a whole lot of evidence to support this. I have never had the "lean" Codeine/Promethazine syrup but i have taken both of them together in pill form many times as they are both OTC here. It's a good combo when i have nothing stronger but i certainly wouldn't pay outrageous prices for a bottle of Codeine/Promethazine syrup.
