OCD responds specifically to drugs that inhibit synoptic re-uptake of serotonin.

These include:

SSRI's (Selctive Serotonin Re-uptake Inhibitors):

Higher doses of SSRI's than those used for depression may be needed to effectively treat OCD.

Clomipramine

Usually reserved for people in whom trials of SSRI's have been ineffective.

The therapeutic response to drug treatment in OCD increases gradually over weeks and months - benefits continue to occure for at least 6 months.  Clinical trial of a particular medication should involve maximum tolerated dose for at least 12 weeks.

For many people treatment may need to be continued indefinitely.