The problem of weight gain did not surface when clinical trials were being conducted on specific drugs, because it requires prolonged usage for noticeable increase in weight. In some cases, weight gain only occurred after 6 months of use.
Unfortunately, the concern on weight gain was not only limited to SSRI antidepressants. The same problem surfaced in two other classes of antidepressant: Tricyclics and MAO iInhibitors. The specific drugs to cause weight gain include Elavil & Tofranil and Parnate & Nardil respectively. For these medications, weight gain can be observed in both short and long term usage.