What is a Transdermal Patch?
A transdermal patch (TP) is an adhesive drug delivery device applied to the skin that releases medication at a controlled rate for systemic absorption through the skin and into the bloodstream, bypassing gastrointestinal metabolism and providing sustained therapeutic drug levels. 1, 2
Core Mechanism and Design
- Transdermal patches are non-invasive dosage forms designed to deliver a specific dose of medication through the skin barrier into systemic circulation 1, 3
- The patch consists of an adhesive component that is critical to safety, efficacy, and quality—ensuring the device remains in contact with skin for the intended duration 4
- Drug delivery occurs through skin penetration, avoiding first-pass hepatic metabolism that occurs with oral medications 1, 2
Key Advantages Over Other Routes
- Avoids gastrointestinal issues: Eliminates problems with enzymatic digestion, drug hydrolysis in acidic stomach environment, and GI irritation 1, 3
- Stable drug levels: Provides more consistent serum concentrations compared to oral or injectable routes that produce peaks and troughs 5
- Patient convenience: Reduces dosing frequency and improves adherence compared to multiple daily oral doses 3
- Bypasses liver metabolism: Drug enters systemic circulation directly without initial hepatic processing 2
Common Medical Applications
Transdermal patches are FDA-approved and widely used for multiple therapeutic areas:
- Pain management: Fentanyl patches for chronic pain (25 μg/h dosing applied 12-14 hours before surgery in perioperative settings) 6
- Hormone replacement: Testosterone patches and contraceptive patches (estrogen/progestin combinations) 6
- Cardiovascular: Nitroglycerin for angina pectoris 1, 3
- Smoking cessation: Nicotine patches 1
- Hypertension: Clonidine patches 1
Important Clinical Considerations
- Adhesion is critical: The FDA Drug Quality Reporting System has received numerous reports of "adhesion lacking" for transdermal systems—proper skin preparation and application site selection are essential 4
- Skin reactions: Local adverse effects include contact dermatitis, hyperpigmentation, discharge, and discomfort at application sites 6
- Transfer risk: Gel and patch formulations can inadvertently transfer medication to other individuals through prolonged skin contact 5
- Application sites: Patches are typically placed on non-sun-exposed areas such as the abdomen, upper torso, upper outer arm, or buttocks 6
Monitoring and Safety
- For hormone patches (testosterone), peak serum levels occur 6-8 hours after patch application, and levels can be measured at any time once steady state is achieved 5
- Transdermal fentanyl reaches constant concentrations without evidence of respiratory depression when properly dosed 6
- Patches should remain undisturbed for the prescribed duration (typically 24-72 hours depending on formulation) to ensure adequate drug delivery 6