Secondary Endpoint: Impairment caused by allergic symptoms
The ANCOVA indicated significantly more improvement in the OLP group (adj. M = 1.39; SE = 0.16) than in the TAU group (adj.M = 1.93; SE = 0.15), F (1, 71) = 6.445, p = .013, ɳ²p = .083 (see Figure 3c and Table 2), reflecting a medium to large effect (f = .301). Impairment at baseline was also significantly related to impairment at T2, F (1, 71) = 14.689, p < .001, ɳ²p = .171.

Insert Figure 4 here.

Effects of Medication Use

The treatment by medication use ANCOVA indicated that medication use had no significant effects on the reduction of symptom severity, F (2, 67) = 2.386, p = .100, ɳ²p = .066, nor did it interact with the effects of treatment, F (2, 67) = 0.266,p = .767, ɳ²p = .008. For symptom frequency, medication use also had no significant effects on symptom improvement, neither in terms of a main effect, F (2, 67) = 1.983, p = .146, ɳ²p = .056, nor in interaction with the effects of treatment, F (2, 67) = 2.126, p = .127, ɳ²p= .060. For the secondary outcome, there was a significant main effect of medication use on improvement of impairment, F (2, 67) = 4.189,p = .019, ɳ²p = .111, with the least impairment at T2 in people who did not take any medication (M = 1.02;SD = .82) as compared to people who regularly took medication (M = 2.04; SD = 1.32) and people who took medication on demand (M = 1.73; SD = .88). The treatment by medication use interaction was not significant, F (2, 67) = 1.532, p = .224, ɳ²p = .044.

Associations of symptom improvement with additional variables

In Table S1 in the supplement, we present the correlations of symptom burden with particpants’ expectations regarding placebo treatment as well as the extent to which participants felt informed about placebos. The results indicate that expectations were not significantly associated with symptom burden after taking placebos (T2 or T3, respectively). The extent to which participants felt informed about placebos, however, was significantly associated with lower symptom frequency and lower symptom-related impairment in the OLP group.

Adherence and side effects

In the OLP group, 74% of the participants reported that they always took the placebos as prescribed, and 26% said they mostly did so. One participant in the OLP group reported abdominal pain as a side effect after taking the placebo. During the switch-over, one participant from the control group also reported that side effects had occurred during the first week of intake: The person reported gastrointestinal problems, problems falling asleep, itching of the skin and itching in the mouth as side effects.