From the FDA Drug Label
The oral form should supplant the injectable as soon as practicable. In the absence of bioavailability studies establishing bioequivalence between these two dosage forms the following guidelines for dosage are suggested. For an initial approximation of the total daily dose required, the parenteral dose administered in the preceding 24 hours may be used Since this dose is only an initial estimate, it is recommended that careful monitoring of clinical signs and symptoms, including clinical efficacy, sedation, and adverse effects, be carried out periodically for the first several days following the initiation of switchover. In this way, dosage adjustments, either upward or downward, can be quickly accomplished Depending on the patient’s clinical status, the first oral dose should be given within 12 to 24 hours following the last parenteral dose.
The patient should have oral haloperidol coadministered for at least 12 to 24 hours after the last parenteral dose of haloperidol depot 80mg, as this is when the first oral dose should be given.
- The exact duration of coadministration is not explicitly stated, but it is implied to be several days to allow for monitoring and dosage adjustments. 1
From the Research
When transitioning from oral haloperidol 8mg daily to haloperidol depot 80mg, oral haloperidol should be co-administered for approximately 7-10 days after the first depot injection. This recommendation is based on the most recent and highest quality study available, which discusses the pharmacokinetics of long-acting antipsychotic medications, including haloperidol decanoate 2. The overlap period ensures therapeutic blood levels are maintained while the depot formulation gradually reaches steady state. After the initial depot injection, the oral medication should be gradually tapered rather than abruptly discontinued to minimize the risk of withdrawal symptoms or relapse. The depot injection typically takes about 3-7 days to begin releasing significant amounts of medication, with peak plasma levels occurring around 6-7 days post-injection. Subsequent depot injections are usually administered every 4 weeks. This overlap strategy is necessary because haloperidol decanoate (the depot form) is a prodrug that requires time for hydrolysis to release the active medication. Patients should be monitored during this transition period for both efficacy (ensuring symptom control is maintained) and side effects (as total haloperidol exposure may be temporarily higher during the overlap).
Some key points to consider during this transition include:
- The time to reach steady-state for haloperidol decanoate is a function of the absorption rate, and the concentration at steady-state is a function of the elimination rate 2
- The clinical pharmacokinetics of haloperidol decanoate have been extensively studied, and the relationship between plasma concentrations and clinical effects is well understood 3
- The use of haloperidol decanoate has been shown to be effective in managing schizophrenia, with significant improvements in symptoms and behavior compared to placebo 4
- However, the choice of depot medication should be individually tailored, and patient preference should be taken into account 4
It's also important to note that the evidence from older studies, such as those from 1985 and 1986, may not be as relevant to current clinical practice, and should be considered in the context of more recent research 5, 3. Overall, the key consideration is to ensure a smooth transition from oral to depot haloperidol, with minimal disruption to the patient's treatment regimen and minimal risk of adverse effects.