Which characteristic is essential for a drug to be delivered transdermally?

For a drug to be delivered transdermally, it is essential that it does not cause histamine release. This is crucial because histamine release can result in local irritation and an inflammatory response, which would hinder the drug's ability to effectively permeate the skin and reach systemic circulation. A transdermal drug must not provoke significant irritation to ensure it can be used safely and effectively over an extended period.

Transdermal delivery relies on the drug's ability to penetrate the skin barrier without causing adverse reactions. While characteristics such as molecular weight and solubility play a role in the transdermal absorption process, they are not as critical as the drug's potential to cause histamine release. Additionally, drugs suitable for transdermal delivery typically need to be effective at low doses, allowing them to be formulated in patches or gels where high daily doses are impractical.