Transdermal drug absorption allows medications to enter the body through the skin, offering an alternative to oral or injectable routes. Active ingredients pass from the skin’s surface into the bloodstream to exert their therapeutic effects. This approach is useful for treatments requiring sustained drug release or for individuals who have difficulty taking medications by mouth.
The Skin’s Role in Absorption
The skin acts as a protective barrier against external elements. Its outermost layer, the stratum corneum, is primarily responsible for regulating the entry of substances. This layer consists of dead skin cells tightly interlocked with lipids, forming a barrier that restricts the passage of most molecules. For drugs to be absorbed, they must penetrate the stratum corneum to reach deeper layers.
Once past the stratum corneum, drugs diffuse through the viable epidermis and into the dermis, which contains blood vessels and lymphatic vessels. These vessels then transport the medication into the systemic circulation. This process, known as passive diffusion, is the most common mechanism for transdermal drug absorption, driven by a concentration gradient from the application site to the bloodstream. Hair follicles and sweat glands can also provide pathways for drug penetration.
Drugs Commonly Absorbed Through Skin
Many medications are formulated for transdermal delivery, providing steady, controlled release. Pain relievers, such as fentanyl and buprenorphine, are administered via patches for severe or chronic pain. Lidocaine patches are also used for localized pain relief.
Hormone therapies use transdermal routes, including estrogen patches for hormone replacement therapy and testosterone gels or patches for conditions like hypogonadism. These formulations help maintain consistent hormone levels. Nicotine patches are widely used in smoking cessation programs, releasing nicotine to reduce withdrawal symptoms.
Medications for motion sickness, like scopolamine, are available as patches to prevent nausea and vomiting. Certain cardiovascular drugs, such as nitroglycerin, are delivered transdermally to treat angina pectoris. Other examples include clonidine for high blood pressure, and rivastigmine and donepezil for Alzheimer’s disease.
What Affects Skin Absorption?
Several factors influence drug absorption through the skin, including drug properties and skin characteristics. Drugs with smaller molecular sizes generally penetrate the skin more easily than larger molecules. Lipid solubility is also important, as the stratum corneum is rich in lipids; fat-soluble (lipophilic) drugs tend to cross this barrier more readily than water-soluble (hydrophilic) ones.
The drug’s concentration in the applied formulation also impacts absorption, with higher concentrations typically leading to greater penetration. Skin characteristics, such as thickness, vary across the body; thinner skin areas allow for easier drug permeation. Skin hydration can increase permeability, while compromised skin integrity due to cuts, burns, or diseases can significantly enhance absorption.
Blood flow to the skin also plays a role; increased blood flow, often due to higher temperatures, can promote absorption. The drug’s specific formulation (whether it’s a patch, gel, or cream) and the presence of chemical enhancers can also modify the skin’s barrier properties to improve drug penetration.
Applying and Using Skin-Absorbed Medications Safely
Safe and effective use of skin-absorbed medications requires proper application and adherence to instructions. Before applying a patch or topical medication, the skin area should be clean, dry, and free of lotions, oils, or powders. It is generally recommended to avoid applying medications to broken or irritated skin.
Patients should follow specific instructions regarding the application site, as different medications may have recommended areas like the upper chest, upper arm, or hip. Rotating application sites prevents skin irritation. After application, ensure the patch is firmly attached or the gel has dried to prevent accidental transfer to others.
Used patches often contain residual medication and should be disposed of carefully, typically by folding them sticky sides together and placing them in a secure trash receptacle, away from children and pets. Accidental exposure to these medications, particularly for children or pregnant individuals, can lead to serious side effects. Always consult a healthcare provider for any questions or concerns regarding transdermal medications.