3.2.3 Interaction due to changes in drug transport mechanisms. (indirect meehanism) A number of drugs whose actions occur at adrenergic neurons can be prevented from reaching those sites of action by the presence of other drugs. Thus the uptake of Guanethidine and related drugs (Guanoclor, Bethanidine, Debrisoquine, etc.) is blocked by ( Chlorpromazine, Haloperidol, Thiothixene, ) a number of indirectly- acting Sympathomimetic amines and the Tricyclic antidepressants so that the antihypertensive effect is prevented..