Drug tolerance is a reduced reaction to a drug. High concentrations of a substance which constantly binds the membrane-associated receptor is a main cause of the drug resistance. Drug desensitizes receptor decreasing its activation. As a result, the signalling pathways that are associated witj the receptor are not affected by the drug. Altered receptor density and modified receptor firing rate are other mechanisms leading to the drug tolerance. As a result, plasma membrane that regulates the functioning of receptors is an organelle responsible for the drug tolerance.