There is good Level 1 evidence for all of the SSRIs in OCD (citalopram is off-label for OCD in Canada, although it is indicated in the United States and Europe). The tricyclic antidepressant clomipramine similarly has Level 1 evidence for efficacy, and historically has often been considered the most effective pharmacotherapy option, but it is relegated to second–line status because it has a more challenging side-effect profile. All of these medications have the advantage of working on the common comorbid mood and anxiety disorders.
There are two major differences in how these medications are used to treat OCD versus depression:
- Dosing is generally most effective at the upper end of the tested dose range for OCD, which can be higher than the dose range tested for depression.
- There is a longer therapeutic lag before benefits are seen in OCD—generally six to 10 weeks compared with two to four weeks for depression.
It is generally best to discuss the target dose with the patient, stressing that the patient should aim for the upper end of the dose range or until significant side-effects occur, after which it is important to allow at least six to 10 more weeks to assess response. For this reason, drug trials in OCD typically require 12 weeks or more (Fineberg et al., 2015) (see Table 2 for medications and dose ranges).