Olanzapine 20mg Can Be Given as a Single Nighttime Dose
Olanzapine 20mg should be administered as a single daily dose at night rather than divided throughout the day. 1
FDA-Approved Dosing Schedule
The FDA label explicitly states that olanzapine should be administered once daily without regard to meals, with dosing ranges of 5-20 mg/day for schizophrenia and bipolar disorder. 1 There is no recommendation in the official prescribing information to divide the daily dose, even at the maximum approved dose of 20mg. 1
Pharmacokinetic Support for Once-Daily Dosing
Olanzapine has a half-life that supports once-daily administration, as demonstrated in multiple clinical trials where standard doses (5-20 mg/24 hours) were given once daily in the evening with blood levels measured 12 hours later showing therapeutic concentrations. 2
Research confirms that 80% of patients receiving standard doses (5-20 mg/24 hours) as a single daily dose achieved serum concentrations within the therapeutic range of 22-146 nmol/L at 12 hours post-dose. 2
Even at higher doses (30-40 mg/day), olanzapine demonstrated dose-proportional pharmacokinetics when administered once daily, supporting the appropriateness of single-dose administration. 3
Clinical Trial Evidence
All major efficacy trials establishing olanzapine's effectiveness used once-daily dosing regimens, with doses ranging from 5-20 mg/day administered as a single dose. 4, 5
The North American Olanzapine Trial, which established therapeutic plasma concentration thresholds, used once-daily dosing with blood samples obtained 10-16 hours after the last dose. 6
Practical Nighttime Dosing Advantages
Nighttime administration is specifically recommended because olanzapine commonly causes sedation and drowsiness, making evening dosing preferable to minimize daytime impairment. 7
When used in combination with fluoxetine, the FDA label explicitly states administration should occur "once daily in the evening," reinforcing the appropriateness of nighttime dosing. 1
Important Caveats
The maximum approved dose is 20 mg/day for monotherapy in schizophrenia and bipolar disorder; doses above this have not been systematically evaluated for safety in standard clinical practice. 8, 1
For elderly patients, lower starting doses (2.5-5 mg) are recommended, but once-daily administration remains appropriate even in this population. 7, 8
Monitor for orthostatic hypotension, particularly after the first dose, as this is a common side effect that may be more pronounced with nighttime dosing when patients subsequently stand. 7