Surfaces of drug interaction. Axes x and y report the inhibition coefficients h (dosage) for the two drugs; the z-axis reports the percentage of the flux through the modulated reaction after drug treatment calculated with L1 formulation of MOMA with respect to the untreated value (left panels a, d and g), or the relative deviation, η(h), of the effect induced by the drug combination from the linear superposition of the effects due to the single drugs as expressed by (11) (middle column, panels b, e and h). The right panels (c, f and j) show the calculations made with MOMA based on the L2-norm. The higher smoothness of the surface makes this formulation more reliable: however the surfaces obtained with our method (L1-norm) well reproduce the main characteristic of the synergism between the drugs. In the first two rows inhibitory synergisms are shown (τ < 1), whereas the third row is an example of activating synergism (τ > 1). Notice that for the sake of readability, in the plots of the last row, both x and y axes have been inverted.