The use of toxic chemical agents is hazardous for a biological system but there are times when we have to use them in order to obtain desirable results as medicines, pesticides, preservatives and flavouring agents.

In therapeutic practice where the chemical agents are to be used for curative eliminating disease causing and other unwanted organisms knowledge of threshold dose, the effective dose and the lethal dose of a chemical agent is essential. If the effective dose of the chemical agent is fairly close to the lethal dose the dangers of over-dosing are obvious and the relative margin of safety becomes pretty low.

On other hand, if the effective dose at which the desired responses are obtained, is fairly low while the lethal dose is very high, the dangers of over-dosing are negligible. The relative margin of safety, therefore, can be defined as the ratio between the effective dose and the lethal dose. With reference to LD50 and ED50 the margin of safety which is also termed as therapeutic margin shall be given by:

The greater is this ratio, the safer is the chemical. As this ratio approaches close to unity risks of overdosing multiply and the margin of safety decreases. In practice of clinical medicine the ratio between the lethal dose and effective dose is termed as therapeutic index and is usually calculated with reference to LD, to ED99 i.e., the dose of the chemical agent which is effective in about 99% cases whereas being lethal in only 1% individuals. Therefore,

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The therapeutic index of penicillin is over 100 which means that the drug is relatively safe while the, therapeutic index of digitalis is as low as 1.5 to 2.0 meaning thereby that the margin of safety is very narrow and the drug should be administered with great care. In Fig. 21.9, the therapeutic dose response curve (A) and the toxic dose response curve (B) of the same chemical agent has been plotted on the same scale (hypothetical case).

The threshold of toxic action starts much after the effective dose required to produce a favourable response in cent per cent cases. The drug shall have a large margin of safety and a much larger therapeutic margin. Now if curve (B) has to shift to a position near curve (A), the margin of safety shall decline while the therapeutic margin shall also shrink.