The Board of Appeals today approved Boston's fourth medical-marijuana dispensary at 50 Clapp St., off Massachusetts Avenue in Dorchester. Because of its location near the Methadone Mile, the proposed operator agreed to set a minimum purchase price of $40 to discourage any nearby addicts thinking of putting together enough money to buy a joint.

"It's almost like the anti-nip policy if you're a liquor store," Mike Ross, attorney for Natural Selections, told the board.

Natural Selections already has permission to run dispensaries in Watertown and Fitchburg.

The board's approval came with a condition that the company would have to come back before the board for approval should it seek to move into recreational marijuana if the state ever releases regulations actually allowing that. Ross added Natural Selections would agree to prohibit on-site consumption of its wares, which would rule out a marijuana cafe.

Unlike the other two dispensaries approved by the board - on Milk Street downtown and on Newbury Street in the Back Bay - nobody spoke against the proposal. The mayor's office and the office of City Councilor Frank Baker (Dorchester) supported it.

The board has also approved a dispensary on McClellan Highway in East Boston. On Wednesday, the City Council could consider whether to formally "not disapprove" a dispensary on VFW Parkway in West Roxbury.