Fig. 2. Reductions in the shivering threshold (compared with the control day) for the Nefopam, Alfentanil, and two-drug combination (Combi 2) days. Also shown is the expected reduction for the combination of nefopam and alfentanil assuming an additive effect, calculated as the sum of the individual effects of nefopam and alfentanil (Nef & Alf). Two-way repeated-measures analysis of variance did not show a significant interaction between the effects of the two drugs on the shivering threshold (  P = 0.55). The effects were therefore additive, and the combination of the two drugs produced the effect expected based on their individual contributions.