A phenothiazinium compound of Formula (I): wherein: A and B each independently is in which R′ and R″ each independently is a linear, branched or cyclic hydrocarbon group, or R′ and R″ together with the N atom to which they are attached form an optionally substituted 5-, 6- or 7-membered ring; and where XP− is a counteranion and P is 1, 2 or 3; except for the compounds in which A and B are both either —N(CH3)2 or —N(CH2CH3)2 for use in a treatment that requires removal, deactivation or killing of unwanted tissues or cells.