Tolerability of Icelandic Moss Lozenges in Upper Respiratory Tract Diseases – Multicentric Drug Monitoring Study with 3,143 Children Background and Objective: Icelandic moss is a medical herb with therapeutic efficacy in inflammatory airway diseases. Although preparations based on the extract of this lichen have been marketed since the 19th century, their tolerability has not yet been investigated systematically. Patients and Methods: The tolerability of Isla-Moos® and Isla-Mint® lozenges and the changes of clinical symptoms during treatment were investigated in a post-marketing surveillance study performed in 300 predominantly pediatric practices in Germany. The preparations contained 80 and 100 mg, respectively, of an aqueous Icelandic moss extract. 3,143 children between 4 and 12 years of age who suffered from upper respiratory tract diseases received Isla-Moos® (n = 1,848) or Isla-Mint® (n = 1,295) over a period of 1–2 weeks; 61% of the children received 4–6 lozenges per day. Tolerability was evaluated by assessing adverse drug reactions (ADRs). Treatment success was assessed globally by the investigators and the patients’ parents. The parents were also asked to rate their children’s symptoms before and after treatment. Results: During the period of observation, 73 adverse events, most of which were related to the basic disease, were reported in 57 children (1.8%). In 6 children (0.2%) and 7 events (2× itching, 2× nausea, 1× abdominal pain, 1× heartburn, 1× burning in the mouth) a causal relationship with Icelandic moss extract could not be excluded, so that they were evaluated as ADRs. All ADRs subsided spontaneously during the period of observation. The incidence of ADRs was 1 event in 3,008 (95% confidence interval: 1,504–7,019) treatment days. As regards clinical symptoms, 39% of the children were fully recovered and another 55% were improved by the end of the period of observation. Conclusions: During treatment of 4- to 12-year-old children with Isla-Moos® and Isla-Mint® lozenges only isolated, trivial, and transient adverse drug reactions were observed. The results thus confirm the good tolerability of both drugs and support a favorable risk-benefit assessment.