Which route is mainly used to achieve a local effect?

The correct answer is cream, as this dosage form is specifically designed to be applied topically to the skin or mucous membranes. Creams allow for local effects by providing a direct application of medicinal ingredients to the area needing treatment. When applied, the active ingredients in a cream penetrate the outer layer of skin or mucous membranes, delivering therapeutic effects directly at the site without significant systemic absorption.

While injections can also provide localized treatment in certain contexts (like local anesthetics), they generally are associated with a more systemic effect due to the way they enter the bloodstream. Ointments, similar to creams, are used for local effects, but they tend to be thicker and greasier, which might limit their application in certain areas. Suppositories are typically used for systemic effects or treating conditions within the rectal or vaginal areas, and while they can provide localized treatment in those specific areas, they are not primarily categorized for local application like creams are.

Hence, the use of creams stands out as the preferred route for achieving localized effects effectively.