The additive model assumes unspecific effects of equal size in both the placebo and drug arm or groups of studies or experiments. The additive model assumes unspecific effects of equal size in both the placebo and drug arm or groups of studies or experiments. The interactive model, however, suggests that drug-specific effects interact with the placebo responses and will result in unequal placebo effects in the two study arms or experimental groups. (Modified from Enck et al., 2013, with permission.)‏ Manfred Schedlowski et al. Pharmacol Rev 2015;67:697-730 Copyright © 2015 by The American Society for Pharmacology and Experimental Therapeutics