4.4 Antipsychotic injections for immediate effect
Some antipsychotics are available as intramuscular injections intended for rapid control of violent or dangerous behaviour associated with schizophrenia or manic episodes in bipolar disorder; they should be used only if oral therapy is not appropriate. Antipsychotics licensed for use by intramuscular injection for such emergency use include:
- Chlorpromazine hydrochloride
In contrast, intramuscular injection of zuclopenthixol acetate is licensed for initial treatment of acute psychosis (including mania) and for exacerbation of chronic psychosis, where 2–3 days’ duration of effect is needed; it is not licensed for emergency treatment of acutely disturbed or violent behaviour.
The intramuscular dose of some antipsychotics, eg haloperidol, is considerably lower than the oral dose because intramuscular injections are not affected by incomplete gastrointestinal absorption or first-pass metabolism. Rapid transfer of the antipsychotic to the blood from the intramuscular injection site increases the risk of adverse effects such as excessive sedation, extrapyramidal side effects, postural hypotension and respiratory difficulty.
Factors which increase risk
Simultaneous use of other sedative drugs such as benzodiazepines increases the risk of respiratory depression, hypotension, bradycardia, and death.
Physical activity increases blood flow to the injection site, which increases the rate of absorption of the antipsychotic and thereby exposes the patient to more intensive effect.
To avoid excessive exposure, the dose of intramuscular antipsychotics should be determined carefully. The patient should be closely monitored (blood pressure, pulse, breathing rate and level of consciousness) especially for four hours after injection.
An intramuscular antipsychotic injection for immediate effect should never be used for maintenance treatment; licensed recommendations on the frequency of dosing and duration of use should not be exceeded.
These antipsychotics are formulated for intramuscular injection and, therefore, inadvertent intravascular injection must be avoided; advice in the summaries of product characteristics on injection technique must be followed.
If it is necessary to use an antipsychotic and a benzodiazepine, it is best to administer the two drugs at least one hour apart and the patient’s level of consciousness and vital signs should be monitored closely.
This learning module discusses noteworthy risks for antipsychotics. Summaries of product characteristics and the BNF should be consulted for a fuller account of the adverse effects and warnings for individual antipsychotics.
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