March 7, 2020

In a January 28, 2020 Drug Safety Communication entitled “FDA strengthens warning that untreated constipation caused by schizophrenia medicine clozapine (Clozaril) can lead to serious bowel problems,” the FDA stated that it is “strengthening an existing warning that constipation caused by the schizophrenia medicine clozapine (Clozaril, Fazaclo ODT, Versacloz, generics) can, uncommonly, progress to serious bowel complications. This can lead to hospitalization or even death if constipation is not diagnosed and treated quickly. Constipation is a frequent and known side effect of clozapine, but serious and fatal events continue to be reported.”

Clozapine is a medicine that has been used for more than 40 years to treat schizophrenia in patients whose symptoms are not controlled with standard treatment. Symptoms of schizophrenia include hearing voices, seeing things that are not there, and being suspicious or withdrawn. Clozapine is also effective in reducing the risk of suicidal thinking and self-harm in patients with schizophrenia or schizoaffective disorder. It is sold under the brand names Clozaril, Fazaclo ODT, and Versacloz, and as generics by many different drug companies.

Clozapine affects how the intestines function in the majority of patients, producing effects ranging from constipation, which is a common occurrence, to serious but uncommon bowel problems, including complete blockage of the bowels. The FDA found that because of the way clozapine works this risk is greater with clozapine than with the other schizophrenia medicines in its drug class. The risk is further increased at higher doses of clozapine and when it is co-prescribed with a type of medicine called anticholinergics, which can slow the movement in the intestines, and other medicines that cause constipation, including opioids. Many different kinds of medicines have these anticholinergic effects.

The FDA identified 10 cases from 2006 to 2016 describing constipation that progressed to serious bowel problems resulting in hospitalization, surgery, or death (five deaths). The total daily dose of clozapine administered ranged from 200 mg to 600 mg, with a median daily dose of 400 mg. The time to onset of serious bowel events ranged from 3 days to 6 months, with a median of 46 days. These cases included death of the colon wall (necrotizing colitis), blockage of blood flow to the intestines (intestinal ischemia), death of part of the intestine (intestinal necrosis), and abdominal distention leading to bowel obstruction (volvulus). This number includes only cases submitted to the FDA or those found in the medical literature, so there may be additional cases about which the FDA is unaware.

Common side effects of clozapine in addition to constipation include dry mouth, drooling, drowsiness, lightheadedness, shaking or tremor, and blurred vision. In 2018, an estimated 782,000 clozapine prescriptions were dispensed from U.S. outpatient retail pharmacies, which was a 2.3% decline from 2016 (800,000 clozapine prescriptions).


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