Antidepressants raise the neurotransmitter levels very quickly (serotonin/noradrenalin) but therapeutic effect is seen a lot later than the pharmacological effect. The theory is that stress damages the neurons in the brain. The body reacts to a perceived threat and increases levels of stress hormones, if these levels return to baseline levels quickly there isn't an issue but if they stay high for long periods then it starts damaging neurons, and even causing them to die off, which would also explain why the hippocampus of patients of depression is often seem to be smaller than those without. So depression and anxiety causes the body to react as if it's under stress. Now there is another chemical acting in the brain and this chemical stimulates the neurons and can promote neuron growth. Levels of this chemical are low in patients with major depression. In studies on rats, scientists have found that serotonin increases the production of this chemical and this gradually stimulates the renewal of neurons and neuron pathways.
