A few drugs can be formulated such that a “patch” containing the drug is applied to the skin. The drug seeps out of the patch, through the skin at a predetermined and controlled rate, and into the capillary bed. The permeated drug is transported by blood streaming to the whole body without the first-pass effect. This route of drug administration is known as transdermal route of drug administration.
Transdermal patches have multiple layers, as follows:
- Innermost adhesive layer (usually with a priming dose): attaches to the skin.
- Rate-controlling micro-pore membrane: controls the rate of delivery to the skin surface. The rate is usually set to a rate that is slower than the slowest rate of absorption from the skin to avoid unnecessary backlog or wastage on the skin surface.
- Main drug reservoir layer: contains the active drug to be delivered.
- Outer backing film: an occlusive supporting layer.
Transdermal patches usually last for 1 – 3 days, sometimes even longer. Although the patches can be applied anywhere over the body, chest, abdomen, upper arm, lower back, buttock, or mastoid regions are preferred. These regions are usually thin-skinned or highly vascular.
Drugs like fentanyl, nitroglycerin, hyoscine, clonidine, estradiol, and nicotine are delivered through skin patches
Advantages of transdermal route of drug administration
Transdermal route of drug administration is becoming increasingly popular due to the following advantages.
1. Provides smooth plasma concentrations of a drug without fluctuations, for a long period.
2. Drug administration through skin avoids the pH variations seen with gastrointestinal transit.
3. Drug reaches the systemic circulation whilst avoiding first-pass hepatic metabolism (though the skin is metabolically active).
4. Self- administration is possible.
5. Drug intake can be stopped at any point by simply removing the transdermal patch.
6. The simplified medication regimen leads to improved patient compliance and reduced side effects as well as inter and intra-patient variability.
7. Transdermal patches are noninvasive, avoiding the inconvenience of parenteral therapy.
8. Equivalent therapeutic effect can be elicited with less amount of dose if given as a transdermal patch as compared to dose of same drug if given orally.
9. Comparable characteristics with intravenous infusion.
10. Can be route of drug administration of choice in patients who are nauseated or unconscious.
Disadvantages of transdermal route of drug administration
1. The drug, the adhesive, or excipients in the patch formulation may cause rashes, local irritation, erythema, or contact dermatitis.
2. Only drugs with a lipophilic character can effectively cross the stratum corneum and hence the drugs must have some desirable physicochemical properties for penetration. Drugs with a hydrophilic structure will not be able to reach the systemic circulation unless modified to some suitable form.
3. Only potent drugs are suitable candidates for transdermal patch because of the natural limits of drug entry imposed by the skin’s impermeability.
4. Doses of only 5mg or less can be administered in a day.
5. The barrier function of the skin changes from one site to another on the same person, from person to person, and with age.
6. The patch may be uncomfortable to wear as adhesives may not adhere well to all types of skin.
7. Transdermal drug delivery system cannot achieve high drug levels in blood/plasma.
8. Ionic drugs cannot be delivered using transdermal route.
9. Patch may fall-off unnoticed.
- Arunachalam A, Karthikeyan M, Kumar VD, Prathap M, Sethuraman S, Ashutoshkumar S, Manidipa S. Transdermal Drug Delivery System: A Review. Current Pharma Res. 2010; 1(1):70- 81.
- Raj, G. and Raveendran, R. (2019). Introduction to Basics of Pharmacology and Toxicology Volume 1: General and Molecular Pharmacology: Principles of Drug Action. Springer Nature Singapore Pte Ltd.