Transdermal

Transdermal administration refers to delivering a drug or active substance across the skin to reach systemic circulation. It differs from topical therapy, which aims for local effect in the skin. Successful transdermal delivery requires the compound to permeate the epidermal barrier, mainly the stratum corneum, and to be chemically stable in the dermal environment. Key properties include lipophilicity, modest molecular weight (often under 500 Da), and potency sufficient to achieve therapeutic plasma levels given limited dose capacity.

The predominant form is the transdermal patch, which provides controlled, steady release over hours to days. Other approaches include gels, creams, and medicated plasters, as well as technologies to enhance permeation such as microneedle arrays, iontophoresis, and ultrasound. Transdermal systems bypass first-pass liver metabolism, offering more predictable pharmacokinetics and convenient dosing, but absorption can vary with skin condition, site, and integrity, and local irritation or sensitization may occur.

Clinical uses span pain management, nicotine replacement, hormone therapy, and some cardiovascular and psychiatric conditions. Notable products include nicotine patches, fentanyl and buprenorphine patches, rivastigmine and estradiol patches, and scopolamine patches. Advantages include sustained plasma levels, reduced gastrointestinal side effects, and improved adherence. Drawbacks include a limited range of suitable drugs, variable absorption, potential skin reactions, and risk of accidental exposure to others, particularly children or caregivers.

Ongoing research aims to expand drug compatibility, improve permeation technologies, and ensure safety and tolerability. Regulatory review requires demonstration of efficacy and safety, proper labeling, and quality testing for adhesion, dose delivery, and skin compatibility.