The pharmacology of topical analgesics

Pain management of patients continues to pose challenges to clinicians. Given the multiple dimensions of pain--whether acute or chronic, mild, moderate, or severe, nociceptive or neuropathic--a multimodal approach may be needed. Fortunately, clinicians have an array of nonpharmacologic and pharmacologic treatment choices; however, each modality must be chosen carefully, because some often used oral agents are associated with safety and tolerability issues that restrict their use in certain patients. In particular, orally administered nonsteroidal antiinflammatory drugs, opioids, antidepressants, and anticonvulsants are known to cause systemic adverse effects in some patients. To address this problem, a number of topical therapies in various therapeutic classes have been developed to reduce systemic exposure and minimize the risks of patients developing adverse events. For example, topical nonsteroidal anti-inflammatory drug formulations produce a site-specific effect (ie, cyclo-oxygenase inhibition) while decreasing the systemic exposure that may lead to undesired effects in patients. Similarly, derivatives of acetylsalicylic acid (ie, salicylates) are used in topical analgesic formulations that do not significantly enter the patient's systemic circulation. Salicylates, along with capsaicin, menthol, and camphor, compose the counterirritant class of topical analgesics, which produce analgesia by activating and then desensitizing epidermal nociceptors. Additionally, patches and creams that contain the local anesthetic lidocaine, alone or co-formulated with other local anesthetics, are also used to manage patients with select acute and chronic pain states. Perhaps the most common topical analgesic modality is the cautious application of cutaneous cold and heat. Such treatments may decrease pain not by reaching the target tissue through systemic distribution, but by acting more directly on the affected tissue. Despite the tolerability benefits associated with avoiding systemic circulation, topically applied analgesics are associated with application-site reactions in patients, such as dryness, erythema, burning, and discoloration. Furthermore, some adverse events that have been observed in patients may be suggestive of some degree of systemic exposure. This article reviews the mechanisms of action, pharmacokinetics, and tolerability of topical treatments for the management of patient pain.