Sara:
While I agree that noticeable symptom improvement after a single day of antidepressant therapy is unlikely, you may be interested to know that the “conventional wisdom” regarding delayed onset of action is far from certain. At least with respect to SSRI therapy, several reviews of 100+ (previously published) double-blind, placebo controlled studies have concluded that objective measures of depressive symptoms show statistically significant improvement within the first week of therapy, followed by up to five weeks of improvement that diminishes in magnitude. The 4-6 week figure may just be an artifact of the lack of statistical power individual studies possess, such that improvements do not reach the level of statistical significance in small groups of patients until well into treatment.
Further, a delay on the timescale of weeks, if it were present, would suggest changes in gene expression as a prime candidate for the mechanism leading to that delay. It is conceivable that even those drugs that do not produce clinically significant symptom improvement would still induce similar changes in gene expression. It seems to me that prior exposure to the same class of drug might “prime” a brain, thereby truncating any delay when an effective alternative is finally introduced. Pure speculation on my part, but perhaps some food for thought.
