Got this from a website

How do antiobsessional medications work? We don't know the full answer, but we know that it's much more complicated than "not having enough serotonin." For one thing, increasing serotonin levels can actually increase OCD symptoms initially in some patients. Only after weeks of treatment does the body respond to these higher levels by shutting down a significant number of postsynaptic receptors. This process is called downregulation. For reasons we don't yet understand, clinical improvement appears to be correlated with downregulation. So if you start (or increase the dosage of) an antiobsessional medication and your symptoms get worse, this can actually be a good sign that the medication will eventually work for you, and you should try to "tough it out" for the next week or two.