Paediatric population: In a clinical trial with 578 adolescent patients, 12 through 17 years of age, the most common adverse event was headache; this occurred in 5.9% of patients treated with desloratadine and 6.9% of patients receiving placebo.
Tabulated list of adverse reactions: The frequency of the clinical trial adverse reactions reported in excess of placebo and other undesirable effects reported during the post-marketing period are listed in the following table. Frequencies are defined as very common (≥1/10), common (≥1/100 to <1/10), uncommon (≥1/1,000 to <1/100), rare (≥1/10,000 to <1/1,000), very rare (<1/10,000) and not known (cannot be estimated from the available data). (See table.)

Pediatric population: Other undesirable effects reported during the post-marketing period in Pediatric patients with an unknown frequency included QT prolongation, arrhythmia, bradycardia, abnormal behaviour, and aggression.
A retrospective observational safety study indicated an increased incidence of new-onset seizure in patients 0 to 19 years of age when receiving desloratadine compared with periods not receiving desloratadine. Among children 0-4 years old, the adjusted absolute increase was 37.5 (95% Confidence Interval (CI) 10.5-64.5) per 100,000 person years (PY) with a background rate of new onset seizure of 80.3 per 100,000 PY. Among patients 5-19 years of age, the adjusted absolute increase was 11.3 (95% CI 2.3-20.2) per 100,000 PY with a background rate of 36.4 per 100,000 PY. (See Precautions.)
Syrup: Desloratadine syrup, the most frequent adverse events reported in excess of placebo were diarrhea (3.7%), fever (2.3%) and insomnia (2.3%).
Desloratadine tablet, the most frequent adverse events reported in excess of placebo were fatigue (1.2%), dry mouth (0.8%), and headache (0.6%).
Very rare cases (<1:10,000) of hypersensitivity reactions (including anaphylaxis and rash), tachycardia, palpitations, psychomotor hyperactivity, seizures, somnolence, elevations of liver enzymes, hepatitis, increased appetite and increased bilirubin have been reported.
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