Long-Acting Injectable Antipsychotics
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Direct answer
A long-acting injectable antipsychotic converts daily adherence into a monthly event and is offered after two relapses from non-adherence, or by patient choice, in schizophrenia — never as punishment and never for acute agitation. Each depot has a signature rule the exam tests: risperidone microspheres 25-50 mg intramuscularly every two weeks requires oral antipsychotic cover for the first three weeks because release lags; paliperidone palmitate needs no oral overlap, using loading doses of 233 mg into the deltoid on day 1 and 156 mg on day 8 before monthly maintenance; olanzapine pamoate carries a mandatory three-hour post-injection observation for the sedation-and-delirium post-injection syndrome; and haloperidol decanoate is given every four weeks with about a month of oral overlap. Every candidate is established on the oral form first for tolerability, and metabolic monitoring — weight, glucose, lipids — continues unchanged.
What you must remember
- Risperidone microspheres: 25 mg (range 25-50) gluteal every two weeks; oral risperidone must continue for three weeks after the first injection — the classic "oral overlap" answer.
- Paliperidone palmitate monthly: 233 mg deltoid on day 1, 156 mg deltoid on day 8, then 117 mg monthly (39-234 range) — no oral supplementation needed because effective levels appear from day one; deltoid initiation gives the faster rise.
- Olanzapine pamoate: every two to four weeks; post-injection syndrome of excess sedation, delirium and cardiovascular collapse demands a registered three-hour observation window with medical access — the exam's "observation rule".
- Haloperidol decanoate: 50-200 mg every four weeks, roughly equivalent to 10-20 times the daily oral dose per month, with oral overlap for the first month.
- Aripiprazole monohydrate: 400 mg gluteal monthly after tolerability established orally, later extendable to every six weeks.
- Selection logic: two relapses from missed tablets, patient preference, or first-episode with poor insight; not indicated in undiagnosed psychosis, dementia-related behaviour or as a judicial "compliance" tool.
- Continuing care: prolactin (risperidone and paliperidone raise it), metabolic syndrome parameters, injection-site rotation, and documentation of consent — India's Mental Healthcare Act 2017 requires informed consent for long-acting therapy.
A worked initiation
A 27-year-old man with his third relapse of schizophrenia in two years, each traced to stopped tablets within weeks of discharge, agrees to a long-acting injectable. He has tolerated oral risperidone 4 mg, so the conversation is between risperidone microspheres and paliperidone palmitate. Risperidone microspheres 50 mg is chosen; he is told plainly that tablets must continue for the first three weeks — the microspheres release almost nothing initially — and that omitting this overlap is the commonest initiation error in real practice. By week six he attends monthly, and the relapse cycle breaks.
Had paliperidone palmitate been chosen instead, the calendar would read day 1 and day 8 deltoid loading doses, then monthly maintenance with no overlap — a difference he might value if tablet-taking is precisely what he abandons. The contrast between the two — three-week overlap versus day-1 activity — is the single most examined fact in this class, and remembering why (microsphere dissolution lag versus nanocrystal water solubility) makes the fact durable rather than memorised.
Where students slip
Candidates conflate the depots and lose easy marks: the three-week oral overlap belongs to risperidone microspheres alone; the day 1/day 8 loading pair belongs to paliperidone; the three-hour observation belongs to olanzapine pamoate. A second systematic error is indication — using a depot for acute agitation or aggression is wrong because no depot reaches therapeutic levels quickly; acute disturbance is managed with short-acting oral or intramuscular medication. Third, tolerability is established orally before any injectable is given — initiating a depot in a treatment-naive patient is both unsafe and a wrong option, however attractive "ensured compliance" looks in the stem.
Frequently asked questions
Which long-acting antipsychotic requires oral coverage for the first three weeks?
Risperidone microspheres, because significant drug release begins only about three weeks after injection.
How is paliperidone palmitate initiated?
With deltoid loading injections of 233 mg on day 1 and 156 mg on day 8, then monthly maintenance — no oral overlap is required.
What is the post-injection syndrome of olanzapine pamoate?
Rare but serious excess sedation, delirium and cardiovascular collapse from partial intravascular entry, mandating three hours of observed monitoring after every injection.
When is a long-acting injectable indicated?
After relapses attributable to non-adherence, or by patient preference, in maintained schizophrenia — after oral tolerability is proven, and never for acute control.
How is haloperidol decanoate dosed?
Approximately 10-20 times the daily oral haloperidol dose, given intramuscularly every four weeks, with oral supplementation during the first month.