Toxic and beneficial drug effects

The first case applies if toxicity arises simply as an extension of the therapeutic effect. As an example, consider warfarin, an inhibitor of an enzyme necessary in the post-trans-lational modification of blood coagulation factors . While it may help to prevent thrombosis and stroke when used in low amounts, any excess in drug effect will be highly dangerous, leading to things such as spontaneous hemorrhage into the brain. 

Toxicity as an extension of therapeutic action is usually associated with a small therapeutic index, which is simply the ratio of the toxic plasma concentration over the therapeutic plasma concentration. It should be apparent that drugs with a small therapeutic index require the most attention and alertness with respect to variations in metabolism and elimination. Such variations may easily cause the concentration within the body to either exceed the toxicity threshold, or drop below the minimum amount required for the therapeutic effect. Accordingly, in our example, patients receiving warfarin treatment need to have their blood clotting function measured at regular, frequent intervals. 

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