Obligatory Mob references: The Five Families by Selwyn Raab and Complete Idiot's Guide to the Mafia by Jerry Capeci. Also, Donnie Brasco: My Undercover Life in the Mafia by Joseph Pistone for the particular subject.

In a general sense, loosely. Trading in narcotics was technically forbidden by the Mafia, due to the harsh penalties, and initiates were warned that such trades would mean death. Like most such rules it went by the wayside when there was money to be made. Many bosses didn't question where their tribute payments were coming from, and would only execute an offender when he was indicted for drug activities.

Long-term partnerships were somewhat rare. Since drug activities were unsanctioned, connections to foreign drug rings were ad hoc in nature, and would often be a personal connection to a particular soldier or captain rather than the family as a whole. Often mobsters wouldn't actually be involved in the handling of drugs, and would rather finance deals and "protect" independent dealers from other mobsters.

The chief exceptions would involve the Sicilian Mafia. Sicily was for a long time a stopping point for heroin originating in Turkey headed to the US, and its mafia chieftains had a heavy hand in the importation and distribution. American mafiosi such as Carlo Gambino and Joseph Bonnano had began their criminal careers as young men in Sicily, and retained a number of professional and family connections in the old country. The Bonnano family, in particular, was heavily involved at a high level in the so-called "Pizza Connection," in which immigrant Sicilian mafiosi would import heroin in containers of cheese and olive oil, and distribute it across the northeastern US and Canada.