Cetraria islandica is a medicinal 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. 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 paediatric practices in Germany. The preparations contained 80 and 100 mg, respectively, of an aqueous C. islandica extract. A total of 3143 children between 4 and 12 years of age who suffered from upper respiratory tract diseases received Isla-Moos (n = 1848) or Isla-Mint (n = 1295) 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. 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 six children (0.2%) and seven events (two itching, two nausea, one abdominal pain, one heartburn and one burning in the mouth) a causal relationship with C. islandica extract could not be excluded. All ADRs subsided spontaneously during the period of observation. The incidence of ADRs was one event in 3008 (95% CI: 1504–7019) treatment days. By the end of the period of observation 39% of the children were fully recovered and another 55% were improved.