The final destination (internal or external phase of the emulsion) of the drug must be considered when selecting a levigating agent. If the drug will reside in the internal phase (water phase), then the levigating agent should be water soluble or miscible.

When would you use a levigating agent?

In general, you need an emulsifier to decrease surface tension between oil + water. In order to decrease surface tension between a solid + liquid, you will need a levigating/wetting agent.

How do you choose an ointment base?

Many factors determine the choice of an ointment base. These include the action desired, the nature of the medication to be incorporated and its bioavailability and stability, and the desired shelf life of the finished product (1).