Dosages and administration

In Table 10. some of the suggested optimal plasma concentrations for different antidepressant drugs are listed. For nortriptyline, desipramine, imipramine, and amitriptyline there is some evidence for a minimal plasma and concentration necessary for clinical response. An established therapeutic range is available for nortriptyline. Thus, patients with nortriptyline concentrations between 50 and 150 ng/ml seem to do much better. With the other drugs, it is generally thought that the plasma levels reflect a minimal threshold of plasma concentration for clinical response. Below this level patients are less likely to respond and the upper limit indicates that there is increased possibility of the systemic or cardiac toxicity. Bupropion levels between 50 and 100 ng/ml may possibly be the best range. However, it is important that the dose be kept below 450 mg/day because of the possibility of seizures.

Except for nortriptyline and the use of plasma concentrations to obtain a minimal effective level, it is generally the patient's clinical response that dictates dosage adjustments. One difficulty is that some patients with plasma concentrations outside the therapeutic range do respond and many patients with concentrations within the therapeutic range do not. Thus, the dosage needs to be adjusted depending on the individual patients response. Clearly plasma concentration monitoring can be helpful in many situations such as evaluating plasma levels when higher than standard doses are used, assessing toxicity, use in elderly patients or patients with comorbid conditions to evaluate possible drug interactions, or where compliance is questioned. Blood for drug levels is usually obtained for plasma levels during elimination phase which is usually in the morning 12 h after the last dose.

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