previous case, it is possible to find paradoxical situations where an active metabolite causes the drug's effect. In this case, the increase in active metabolite A2 (following the previous example) produces an increase in the drug's effect. It can often occur that a patient is taking two drugs that are enzymatic inductors; one inductor and the other inhibitor ;or both inhibitors, which greatly complicates the control of an individual's medication and the avoidance of possible adverse reactions. An example of this is shown in the following table for the CYP1A2 enzyme, which is the most common enzyme found in the human liver. The table shows the substrates (drugs metabolized by this enzyme) and the inductors and inhibitors of its activity: Enzyme CYP3A4 is the most commonly used substrate in a great number of drugs. Over a hundred drugs depend on its metabolism for their activity and many others act on the enzyme as inductors or inhibitors. Some foods also act as inductors or inhibitors of enzymatic activity. The following table shows the most common: Any study of pharmacological interactions between particular medicines should also discuss the likely interactions of some medicinal plants. The effects caused by medicinal plants should be considered in the same way as those of medicines as their interaction with the organism gives rise to a pharmacological response. Other drugs can modify this response and also the plants can give rise to changes in the effects of other active ingredients. There is little data available regarding interactions involving medicinal plants for the following reasons: False sense of security regarding medicinal plants. The interaction between a medicinal plant and a drug is usually overlooked due to a belief in the "safety of medicinal plants." Variability of composition, both qualitative and quantitative. The composition of a plant-based drug is often subject to wide variations due to a number of factors such as seasonal differences in concentrations, soil type, climatic changes, or the existence of different varieties of chemical races within the same plant species that have variable compositions of the active ingredient. On occasion, an interaction can be due to just one active ingredient, but this can be absent in some chemical varieties or it can be present in low concentrations, which will not cause an interaction. Counter interactions can even occur. This occurs, for instance, with ginseng, the Panax ginseng variety increases the Prothrombin time, while the Panax