Topical Pain Medications
Local anesthetics, nonsteroidal anti-inflammatory drugs (NSAIDs), and the neurolytic substance capsaicin are three commonly used topical pain medications. These topical medications provide localized relief, targeting tissue on which they are directly applied.1
The local anesthetic Lidocaine is approved by the Food and Drug Administration (FDA) in both patch and jelly form. When applied to the skin, it numbs the area underneath it. Lidocaine blocks sodium-gated channels, which have been shown to play a critical role in the initiation and maintenance of many types of pain. Pain models have demonstrated an upregulation of abnormal sodium channels in damaged peripheral sensory nerves.2 In animal studies, inflammatory conditions such as osteoarthritis have also reported abnormal sodium channels that when antagonized reduce pain.3 Roughly 1% to 5% of lidocaine from the patch (a very safe amount) has been shown to be systemically absorbed in healthy patients.4
The NSAID diclofenac was approved by the FDA in both patch and gel form. NSAIDs function peripherally by blocking the enzyme cyclooxygenase, which converts arachidonic acid to inflammatory prostaglandins. The inflammatory prostaglandins sensitize nerve endings to the action of bradykinin, histamine, and other inflammatory factors, making them more likely to transmit pain. Peripherally, NSAIDs halt this inflammatory cascade. After application directly to the knee nine times over a 5-day period, detectable concentrations of diclofenac are seen in the synovial fluid of the knee. Patients with osteoarthritis of the knee and hand treated with topical diclofenac gel reported pain reduction of 51% and 46%, respectively.5 The amount of diclofenac systemically absorbed from the gel preparation is on average 93% less than that of the systemic exposure from oral diclofenac.6
Capsaicin, the last of the three topical pain medications covered in this book, is an active ingredient in chili pepper. It comes in both a cream form and a patch. Its method of action is to deplete—or “burn out”—small peptides in cutaneous nerves that are involved in sending pain transmissions to the brain. Capsaicin is thought to lead to the depletion of substance P in C fibers (C fibers are unmyelinated nerves that transmit pain), resulting in their reduced peripheral excitability. Although initial exposure excites these nociceptor terminals, prolonged exposure leads to their insensitivity to capsaicin as well as other noxious stimuli.
When to Use
Topical medications can be employed for pain in a focal area, when the patient has adverse effects with oral medication, when there is poor compliance with oral medication, or when patients are completely adverse to taking pills. Topical medications can be used with an oral medication or as a stand-alone pain medication.
Lidocaine patches are FDA approved for use in postherpetic neuralgia (PHN). Clinically, people have used the patch to treat a multitude of painful conditions including peripheral neuropathy, post-thoracotomy pain syndrome, postmastectomy pain syndrome, stump pain, and scar neuroma pain.
The topical forms of diclofenac are useful in both musculoskeletal pain (sprains, strains, and contusions) and osteoarthritis.6,7
Capsaicin is FDA approved for the treatment of PHN. Studies have shown it to be useful in osteoarthritis and diabetic neuropathy.8 Clinically, capsaicin—like lidocaine—has been used to treat the above painful conditions.