Switching Medications

If the patient could not tolerate adequate doses of SSRIs or has not responded to SSRI administered in the upper range of the relevant dose, a trial of CMI is recommended (and vice versa). CMI should be given after an adequate work-up that includes an ECG and ruling out ophthamological problems (i.e. closed angle glaucoma). Although no fixed dose studies were carried out, it seems that high doses of CMI are needed in order to attain responses in OCD patients. The titration to these doses should last for 1–3 weeks. If possible, therapeutic drug monitoring should be performed in order to ascertain blood levels (200–500 ng/ml) for the parent drug plus the desmethyl derivative and to avoid side-effects that result from very high (or even toxic) blood levels.

If tolerated, a dose of 200–300 mg/day is considered efficacious in OCD, and this dose would be administered for 10 weeks before determining a lack of response. Caution is necessary if CMI is administered immediately after fluoxetine; in this case, lower initial doses of CMI should be the rule, due to fluoxetine’s long half-life and the fact that it inhibits cytochrome P450 enzymes (thus increasing the availability of CMI). Switching from SSRIs, with shorter half-lives and less inhibition of cytochrome P450 enzymes (such as fluvoxamine and sertraline) to CMI is less problematic. However, the common procedure of slow titration is recommended

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