How is a topical medication different from a transdermal drug?
Both topical (topiceutical) and transdermal medications are applied locally to the skin. However, once transdermal preparations are absorbed through the skin, the bloodstream distributes the medication throughout the body for a systemic effect. To be effective, the transdermal analgesic requires a systemic analgesic concentration. Transdermal drugs provide the same effect, as if the same active ingredient was taken orally. These kinds of patches can be placed to any skin area (according to the product instructions), since the medication will be delivered through the bloodstream to the targeted area in the body. One example is the Durogesic patch (Janssen Pharmaceutica, Titusville, New Jersey).
In comparison, a topical patch produces more local effects. Once the medication penetrates skin, it takes its effect on tissues (muscles, ligaments, tendons, nerves) that lie directly underneath the area where it was applied on the skin application. These medications do not reach the bloodstream, so they do not result in any significant systemic concentration of analgesic. However, topical medications must be used according to the package instructions, because excessive application for extended periods of time over a larger area can promote increased medication penetration, leading to accumulation in the bloodstream, which may cause side effects. Topical patches include but are not limited to the Bengay spa cream (Pfizer, New York), Lidoderm patch (Endo Pharmaceuticals, Chadds Ford, Pennsylvania), and EMLA cream (2.5% lidocaine and prilocaine 2.5%) (AstraZeneca, Wilmington, Delaware).
Topical agents that are delivered directly to targeted tissues under the skin are advantageous over systemic pain medications for several reasons. One reason is that there is a lower risk of unwanted side effects in the body. For example, topical nonsteroidal antiinflammatory agents (NSAIDs) for controlling pain with osteoarthritis and rheumatic diseases were found to be effective and avoided the gastrointestinal adverse effects of oral NSAIDs like peptic ulcers and hemorrhage. Since topical agents do not result in bodywide (systemic) concentrations or lead to drug-drug interactions, these medications can be safely added to an existing pain treatment plan without worry. Patients with chronic pain conditions may be receiving other pharmacological therapies for their comorbid conditions, so the ability to add a topiceutical to their existing regiment is clinically helpful.