Correlation of subjective side effects with plasma concentrations of nortriptyline

Plasma levels of tricyclic antidepressant drugs vary considerably between individuals receiving the same amount of drug. The bearing of this variation on the occurrence of subjective side effects was investigated in 40 psychiatric inpatients with depressive disorders. Plasma levels were determined before and during four weeks of treatment with nortriptyline 50 mg. three times a day and patients were rated for subjective side effects, the assessors being unaware of the plasma levels of the drug.Plasma levels varied widely between individual patients, but in any given patient the plasma level tended to be constant over a period of time. The side effects of nortriptyline diminished significantly with time and were in most cases absent during the fourth week of treatment. There was a significant positive correlation between plasma level of nortriptyline and subjective side effects.The steady-state plasma level of a drug which is metabolized is usually a more important determinant for its effect than dosage, since it reflects the amount of drug available for biological action. Very high plasma levels of nortriptyline should presumably be avoided, since there is no evidence that they are needed for therapeutic effect and they are potentially harmful.