Antipsychotics and antimuscarinics are often given together advantageously and uneventfully, but occasionally serious and even life-threatening interactions occur. These include heat-stroke in hot and humid conditions, severe constipation, adynamic ileus, and atropine-like psychoses. Antimuscarinics used to counteract the extrapyramidal adverse effects of antipsychotics may also reduce or abolish their therapeutic effects. See also antimuscarinics, as many antipsychotics also have antimuscarinic adverse effects.
These drugs have been widely used together with apparent advantage and often without problems. However, be aware that an unspectacular low-grade antimus- carinic toxicity can easily go undetected, particularly in the elderly. Also note that serious problems can sometimes develop, particularly if high doses are used. Consider:
  • warning patients (particularly those on high doses) to minimise outdoor exposure
    and/or exercise in hot and humid climates.
  • being alert for severe constipation and for the development of complete gut stasis,
    which can be fatal.
  • that the symptoms of central antimuscarinic psychosis can be confused with the
    basic psychotic symptoms of the patient.
  • withdrawal of one or more of the drugs, or a dosage reduction and/or appropriate
    symptomatic treatment if any of these interactions occur.
    Some antipsychotics and antimuscarinics prolong the QT interval. For interactions resulting from additive effects on the QT interval see drugs that prolong the QT interval. Remember that many drugs have antimuscarinic adverse effects (e.g. tricyclics).
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