Why are semi-synthetic penicillins more widely used instead of the natural penicillin g?

The several kinds of penicillin synthesized by various species of the mold Penicillium may be divided into two classes: the naturally occurring penicillins (those formed during the process of mold fermentation) and the semisynthetic penicillins (those in which the structure of a chemical substance—6-aminopenicillanic acid—found in all penicillins is altered in various ways). Because it is possible to change the characteristics of the antibiotic, different types of penicillin are produced for different therapeutic purposes.

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