Many substrates are broken down by hepatic cytochrome P450 enzymes. If the breakdown happens too fast, the substrate will not achieve the desired effect. If it occurs too slowly, the concentration of the substrate may rise to a toxic level.

Polypharmacy can constitute one reason for this variability, inhibiting or inducing the catabolism of a substrate; genetic polymorphisms of the catabolic enzyme may be another one. That aside, there may be further possible reasons.

Cypper serves for testing scientific hypotheses on inductions, inhibitions and poly­morphisms of the clinical most important human CYP enzymes. The program uses a rule set deducting possible alterations of the concen­tration of selected subtrates.