Converting Haloperidol Decanoate to Oral Haloperidol
The standard conversion from haloperidol decanoate to oral haloperidol uses a ratio of approximately 10-20 times the daily oral dose to determine the monthly decanoate dose; therefore, to convert back, divide the monthly decanoate dose by 10-20 to estimate the daily oral dose.
Conversion Methodology
Standard Conversion Ratios
The most commonly validated approach uses a 20-fold conversion factor for most patients 1, 2:
- Monthly decanoate dose ÷ 20 = Daily oral haloperidol dose
- Example: A patient on 100 mg haloperidol decanoate monthly would convert to approximately 5 mg oral haloperidol daily
Alternative conversion factors exist depending on clinical context 1, 2:
- Conservative approach: Use a 10-fold factor (monthly dose ÷ 10 = daily oral dose) for patients who may be more sensitive
- Standard approach: Use a 20-fold factor (most commonly validated) 1
- Aggressive approach: Use a 30-fold factor only in select cases 1
Pharmacokinetic Considerations
The decanoate formulation has a half-life of approximately 21 days, which creates important timing considerations 2:
- Plasma haloperidol levels from decanoate reach steady-state after the second injection 1
- When converting to oral, overlap oral therapy for 2-4 weeks while decanoate levels decline to avoid relapse 3, 4
- Plasma levels from decanoate are typically twice as high as equivalent oral dosing, suggesting the need for careful monitoring during conversion 1
Practical Conversion Protocol
Step-by-Step Approach
Calculate the estimated daily oral dose using the 20-fold conversion (monthly decanoate dose ÷ 20) 1, 2
Initiate oral haloperidol immediately at the calculated dose without waiting for decanoate to clear 3, 4
Monitor plasma levels weekly during the first month if available, as decanoate levels will gradually decline while oral levels stabilize 4
Adjust oral dosing based on clinical response during weeks 2-4, as the residual decanoate effect diminishes 3, 4
Expect dose adjustments upward by week 3-4 as decanoate fully clears, potentially requiring 1.5-2 times the initial calculated oral dose 1
Critical Warnings and Pitfalls
Common Conversion Errors
Do not simply stop decanoate and start oral at the calculated dose without overlap, as this frequently leads to relapse during the first month 3:
- Patients converted without adequate oral supplementation during the transition period showed clinical deterioration 3
- The long half-life means therapeutic levels persist for weeks, but decline gradually 2
Monitoring Requirements
Closely monitor for both relapse and side effects during the conversion period 3, 4:
- Extrapyramidal symptoms may initially decrease as total haloperidol exposure declines 1
- Clinical deterioration typically occurs in weeks 2-4 if oral dosing is inadequate 3
- QTc monitoring is prudent, especially if oral doses exceed 5-10 mg daily 5, 6
Special Populations
Elderly or frail patients require lower conversion ratios 5, 6:
- Consider using a 10-fold conversion factor (more conservative) rather than 20-fold
- Maximum daily oral doses should not exceed 5 mg in elderly patients 5, 6
- Start with 0.5-1 mg oral doses and titrate slowly 5
Alternative Consideration
If the conversion is for acute management rather than maintenance therapy, the clinical context matters significantly. For delirium or acute agitation, standard dosing guidelines apply regardless of prior decanoate use 7, 5, 6, starting with 0.5-2 mg doses as needed rather than calculated conversion doses.