Quetiapine is a member of a class of medications known as atypical antipsychotics. It has been approved to treat the symptoms of bipolar disorder as well as those of schizophrenia, and is becoming widely prescribed for a number of off-label psychiatric uses. Since the side-effects of quetiapine can be quite severe and tend to increase as the dosage level increases, it is important to administer the smallest sufficient dose for the shortest possible length of time. The factors that must be adjusted when determining an adequate quetiapine dose include the age of the patient, the condition to be treated, the form of the medication used, impairments in liver or kidney function, and the concurrent use of certain medications.
When using quetiapine to treat the symptoms of schizophrenia, the standard adult quetiapine dose is 300 mg once daily of the extended release tablets, or 25 to 50 mg twice or three times daily for the immediate release tablets. In geriatric patients, the standard initial immediate release dose is 25 mg once daily, although the medication should not be used in patients suffering from dementia. When beginning treatment, the use of the immediate release tablet is preferred, as it allows the clinician to better determine when a dosage increase of 25 to 50 mg is needed. While maximum clinical effect has been reported to be 300 mg per day in most patients, doses as high as 750 mg daily may be used.
The quietiapine dose standard for the treatment of bipolar disorder — either alone or in combination with other medications — is higher than that for treating schizophrenia symptoms. Initially, an immediate release quetiapine dose of 50 mg administered twice daily should be used when treating mania, while a 50 mg dose given once daily should be used to treat depressive episodes. Quetiapine dose adjustments should not exceed 800 mg per day for mania or 600 mg per day for depression.
Several quetiapine drug interactions exist, which require adjustments in dose or the use of alternative medications. Commonly used drugs like phenytoin increases the ability of the liver to metabolize quetiapine due to its effects on the enzyme cytochrome 450, as do many other drugs. If the patient is takeing cytochrome 450 inducers, larger doses of quetiapine may be needed. The cessation of therapy with a cytochrome 450 inducer may result in an overdose if the quetiapine dose is not lowered to compensate. Due to this risk, it may be best to avoid combining quetiapine with any medications that affect the levels of this liver enzyme.