PREPARATIONS: Tablets: 25, 50, 100, 200, 300 and 400 mg. Tablet (Extended Release): 50, 150, 200, 300 and 400 mg
DRUG INTERACTIONS: Phenytoin (Dilantin) and thioridazine (Mellaril) markedly decrease the amount of quetiapine that is absorbed from the intestine and thereby reduces its effectiveness. Therefore, patients taking phenytoin or thioridazine may require higher doses of quetiapine.
Quetiapine can add to the sedating effects of other drugs that sedate. Such drugs include narcotic pain relievers (for example, oxycodone and acetaminophen [Percocet, Roxicet, Tylox, Endocet]), barbiturates, sedatives such as alprazolam [Xanax] and clonazepam [Klonopin], ethanol, and blood pressure drugs that can cause orthostatic hypotension, such as prazosin (Minipress) and terazosin (Hytrin).
Quetiapine is eliminated from the body by an enzyme in the liver called cytochrome P450 3A. There is a concern that drugs that strongly interfere with the enzyme may cause elevated and toxic levels of quetiapine, for example:
- ketoconazole (Nizoral),
- itraconazole (Sporanox),
- fluconazole (Diflucan),
- clarithromycin (Biaxin),
- nefazodone (Serzone),
- verapamil (Calan, Isoptin, Verelan),
- diltiazem (Cardizem, Tiazac, Dilacor)
- St. John's Wort,
- carbamazepine (Tegretol), and
- rifampin (Rifadin).
PREGNANCY AND BREASTFEEDING SAFETY: There are no adequate studies of quetiapine in pregnant women. Studies in animals are inconsistent. Some studies suggest effects on the fetus and others show no effects. Quetiapine should be used in pregnancy only if the physician feels that it is necessary and that the potential benefits justify the unknown risks.
Quetiapine is excreted in the milk of animals during lactation. Although it is not known if it is excreted in human milk, it is recommended that women taking quetiapine not breastfeed.
STORAGE: Tablets should be stored at room temperature, 15 C to 30 C (59 F to 86 F).
DOSING: Immediate release quetiapine usually is taken two or three times daily. Extended release quetiapine is taken once daily. The dose usually is increased slowly over several days or weeks to achieve the desired effect. Quetiapine can be taken with or without food.
The initial dose for bipolar disorder is 50 mg twice daily (100 mg/day) of immediate release quetiapine. The dose can be increased by 100 mg/day to a daily dose of 400 mg/day. Most patients respond to 400-800 mg/day. Doses greater than 800 mg/d have not been studied. The starting dose is 300 mg once daily and the target dose is 400-800 mg once daily when using extended release tablets.
The initial dose for schizophrenia is 25 mg twice daily (50 mg/day) of immediate release tablets. The dose can be increased by 25-50 mg two or three times daily. The target dose is 300-400 mg/day in two or three doses. Patients respond to 150-750 mg/day, and doses greater than 800 mg/day have not been evaluated. The starting dose is 300 mg once daily and the target dose is 400-800 mg once daily when using extended release tablets.
The dose range for treating major depression is 150-300 mg/day of extended release tablets. The starting does is 50 mg in the evening for 2 days increasing to 150 mg in the evening.
DRUG CLASS AND MECHANISM: Quetiapine is an oral atypical antipsychotic drug used for treating schizophrenia and bipolar disorder. Although the mechanism of action of quetiapine is unknown, like other atypical anti-psychotics, it inhibits communication among nerves of the brain. It does this by blocking receptors on the nerves for several neurotransmitters, the chemicals that nerves use to communicate with each other. It is thought that its beneficial effect is due to blocking of the dopamine type 2 (D2) and serotonin type 2 (5-HT2) receptors. The FDA approved quetiapine in September 1997.
Medically reviewed by Eni Williams, PharmD
Reference: FDA Prescribing Information
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