To the Editor: Clinicians seek guidance regarding conversion factors that will allow them to compare novel neuroleptics to existing conventional agents in milligram equivalents. Recently published APA guidelines indicate that a 1–2-mg dose of risperidone equals a 2-mg dose of haloperidol R411559BCEDBBGD, suggesting that haloperidol may require doses twice as high as those of risperidone. However, several lines of investigation support equivalent dosing.

Positron emission tomography findings indicate that risperidone and haloperidol are approximately equivalent in terms of dopamine 2 (D₂) occupancy. Specifically, mean D₂ occupancies for risperidone and haloperidol at 2 mg are 66.3% and 66.7%, respectively (2, 3). While it might be argued that risperidone"s shared serotonin-dopamine antagonism prevents a comparison based simply on D₂ occupancy, there is no evidence to date to suggest that risperidone is different from conventional neuroleptics such as haloperidol in terms of the need to achieve at least 60%–70% D₂ occupancy for antipsychotic response R411559BCEDBGDE-R411559BCEBIIJC. This is in contrast to clozapine, which clearly demonstrates its therapeutic effect at levels of D₂ occupancy below 60% R411559BCEBIIJC, R411559BCECHEJJ.

Comparative data between risperidone and haloperidol, when flexible dosing is employed, provide further support for dose equivalence. In a recent double-blind study of first-episode psychosis R411559BCEBGGAF, individuals were randomly assigned to 4 mg/day of either haloperidol or risperidone. Thereafter, doses of each could be titrated by 2 mg, on the basis of clinical response and side effects, to a maximum of 16 mg/day or a minimum of 2 mg/day. Results indicated that dosing changes over the 6-week trial were comparable for both compounds; at endpoint, the final doses for risperidone and haloperidol were 6.1 mg/day and 5.6 mg/day, respectively.

Comparative data between risperidone and haloperidol, when flexible dosing is employed, provide further support for dose equivalence. In a recent double-blind study of first-episode psychosis R411559BCEBGGAF, individuals were randomly assigned to 4 mg/day of either haloperidol or risperidone. Thereafter, doses of each could be titrated by 2 mg, on the basis of clinical response and side effects, to a maximum of 16 mg/day or a minimum of 2 mg/day. Results indicated that dosing changes over the 6-week trial were comparable for both compounds; at endpoint, the final doses for risperidone and haloperidol were 6.1 mg/day and 5.6 mg/day, respectively.

Evidence involving patients with chronic schizophrenia, although more indirect, also suggests equivalent dosing. Unfortunately, in controlled trials directly comparing risperidone to haloperidol, lower doses of risperidone (i.e., 1–16 mg/day) have been compared to haloperidol doses of 10–20 mg/day R411559BCEBBJGG-R411559BCEDGECG. This has made it virtually impossible to establish whether these two compounds may be equipotent. However, other reports involving neuroleptics in lower doses, including a meta-analysis of 22 randomized, controlled trials, have indicated that for a compound such as haloperidol, the optimal daily dose range is between 3 and 8 mg R411559BCEBICGI-R411559BCEBJCEH. This closely parallels the multiple fixed-dose studies with risperidone indicating that 4–8 mg/day appears to represent the optimal dose range for a similar patient population R411559BCEBBJGG-R411559BCEDGECG.

The recommendation that risperidone and haloperidol be considered equal in terms of dosing must be made with the caveat that these compounds are not necessarily clinically equivalent. Specific advantages have been linked to risperidone"s concomitant 5-hydroxytryptamine antagonism, such as diminished extrapyramidal side effects and improvement in negative symptoms (8–10). At the same time, though, the recommendation means that the results of studies comparing these agents must be revisited and that future investigations must employ comparable doses of each.