How long does Paxil (paroxetine) take to have a therapeutic effect? Why does it not work instantaneously, like benzodiazepines?

4-8 weeks. It takes about 4 to 8 weeks to see the full effects of a particular dose of paxil (paroxetine). This is because the immediate Paxil (paroxetine) effects of increasing serotonin levels in the areas that connect brain cells (synapses) are not correlated with changes in mood or anxiety; instead, receptor changes and nerve growth factors stimulated by weeks of elevated serotonin seem to be.
A couple weeks. Benzodiazepines work by acting directly on the receptors on the cells so you get a more immediate effect. Paxil (paroxetine) acts by making your body by slowing down the reputake of serotonin so the process takes longer to build up the neurotransmitter so there's a noticeable clinical effect.