Urine drug screen false-positives can have quite negative consequences to the patient, on personal and social levels and lead to clinical misdiagnosis.

To report a case of opioid false-positive in urine drug screen (TOX/See BIO-RAD) in a patient medicated with levomepromazine and discuss the implications of our findings.

To stress the importance of excluding cross-reactivity induced by drugs in patients presenting with psychiatric symptoms.

Report of a case and overview of the literature available in MEDLINE.

A 24-year old man was involuntarily committed to our psyhiatric inpatient unit with a diagnosis of manic episode. A rapid urine drug toxicology screen was positive for cannabinoids. In the past he consumed heroin and cocaine. At present, he reports using cannabinoids only. He started risperidone, lorazepam and levomepromazine. In the next days, levomepromazine dosage was increased. During hospitalization there was a suspicion that the patient was consuming drugs. A urine drug screen was performed on day 7 and it was positive for cannabinoids and opioids, which the patient contested. The test was repeated five times, every 2 days, and remained positive. After that a different test, a direct identification test in blood, was performed and it was negative for opioids. Levomepromazine was discontinued. One day after the urine drug screen was repeated and was negative for opioids. This is the first report suggesting opioids cross-reactivity in urine drug screen induced by levomepromazine.

Levomepromazine or it's interactions with other drugs may cause false-positives in urine drug screen.