Background Treatment with antipsychotic drugs has been associated with glucose dysregulation in older outpatients, especially in the early stage of therapy. The underlying mechanism is, however, unclear. The aim of this study was to investigate changes in glucose levels during haloperidol use compared with the use of placebo among older hospitalized patients. Methods This substudy was part of a larger multicenter, randomized, double blind, placebo-controlled clinical trial among hospitalized patients aged 70 years and older who had an increased risk of in-hospital delirium. Patients who were admitted to the Jeroen Bosch Hospital in 's-Hertogenbosch between June 2014 and February 2015 were invited to participate in the study. Participating patients were randomized for treatment and given 1 mg of haloperidol or a placebo twice daily for a maximum of 7 consecutive days (14 doses). Exclusion criteria for this substudy were the use of corticosteroids and changes in diabetes medication. Random blood samples to determine glucose levels were collected before day 1 and on day 6 of the study. Student independent sample t test was used to determine differences in glucose changes between both groups. Results Twenty-nine patients were included (haloperidol, n = 14; placebo, n = 15). The mean glucose level for placebo users was 139.3 mg/dL (SD, 50.1) on day 1 and 140.8 mg/dL (SD, 45.7) on day 6, and the mean glucose level for haloperidol users was 139.9 mg/dL (SD, 71.0) on day 1 and 150.2 mg/dL (SD, 39.1) on day 6. The difference was not statistically significant (P = 0.685). Conclusions Short-term prophylactic use of haloperidol was not associated with changes in glucose levels in older hospitalized patients compared with those given a placebo in this small study.