The possibility should be borne in mind that phenothiazines may: Increase the central nervous system depression produced by drugs such as alcohol, hypnotics, sedatives and strong analgesics.
Antagonise the action of adrenaline and other sympathomimetic agents and reverse the blood-pressure-lowering effects of adrenergic blocking agents such as guanethidine and clonidine.
lmpair: The antiparkinsonian effects of L-dopa; The effects of anticonvulsants; Metabolism of tricyclic antidepressants; The control of diabetes.
Increase the effect of anticoagulants.
Interact with lithium.
Anti-parkinsonian drugs may enhance anticholinergic effects. Phenothiazines may enhance the cardiac-depressant effects of quinidine, the absorption of corticosteroids and digoxin and neuromuscular blocking agents.
ACE lnhibitors, hypotension-producing medications, other: concurrent use with phenothiazines may produce severe hypotension with postural syncope.
Beta-adrenergic blocking agents: concurrent use of beta-blockers, possibly including ophthalmics, with phenothiazines may result in an increased plasma concentration of each medication because of inhibition of metabolism; this may result in additive hypotensive effects, irreversible retinopathy, cardiac arrhythmias, and tardive dyskinesia.
Anticholinergics or other medications with anticholinergic action: concurrent use with phenothiazines may intensify anticholinergic side effects, especially confusion, hallucinations, and nightmares, because of the phenothiazines' secondary anticholinergic effects; medications with anticholinergic effects may potentiate the hyperpyretic effect of phenothiazines, especially when environmental temperatures are high, by preventing sweating as a cooling mechanism; this effect could lead to heat stroke; also, patients should be advised to report occurrence of gastrointestinal problems since paralytic ileus may occur with concurrent therapy.
Antacids, aluminum- or magnesium-containing or Antidiarrheals, adsorbent: Concurrent use of these medications with phenothiazines may inhibit the absorption of orally administered phenothiazines; simultaneous use should be avoided.
Amphetamines: Stimulant effects may be decreased when amphetamines are used concurrently with phenothiazines since phenothiazines produce alpha-adrenergic blockade; also, the antipsychotic effectiveness of phenothiazines may be reduced.