Enzymes affect the speed of reactions predominantly. In some instances in which more than one reaction is possible, enzymes increase the speed of the normally slower reaction enough to make it predominate, while the other possible reaction is effectively suppressed.
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If by injection types you are referring to IM, SQ, IV etc. the
reason can be to prolong the action of the drug, decrease the
possibility of an injection site reaction, or in some instances a
systemic reaction or even anaphylactic shock.