**2. Role of the vehicle and excipients and interaction with the active principles**

A vehicle is defined by the type of preparation (cream, ointment, gel) and the excipients (water, paraffin, propilen glycol); the terms "vehicle" and "excipient" refer to different entities.

Vehicle and excipients deeply influence the velocity and magnitude of absorption and consequently the bioavailability and efficacy. The excipients of the vehicle modulate the effects of partition and diffusion in the stratum corneum.

A lipid preparation that promotes occlusion may enhance the penetration of the drug, but ointments and lipid preparations are not always more powerful than creams. Creams, gels and solutions may be formulated so as to obtain an effect equivalent to that of ointments. Topical corticosteroids of different classes of potency may show the same activity when formulated in different vehicles. A gel preparation of kellin, obtaining better penetration, has demonstrated important results in the treatment of vitiligo. Also transfollicular penetration is influenced by vehicle and excipients; better results are given by lipophilic and alcoholic vehicles. Relevant factors include dimension and charge of the molecules of the solute.
