OTC Topical Analgesic Creams and Gels
OTC topical medications do not control pain or inflammation directly, but rather act by a mechanism called counter-irritation. Pain relief occurs from nerve stimulation and depression. Products in this category include methyl salicylate, camphor, menthol, methyl nicotinate, capsicum, and trolamine salicylate.
The two most commonly used topical analgesics for arthritis pain relief are salicylates and capsaicin. Salicylates, such as Aspercreme, work in the same way aspirin does. Salicylates (Aspercreme) come from aspirin, however unlike aspirin they act by counter-irritation, stimulating pain receptors and desensitizing them. Unlike Voltaren gel (below) they do not inhibit COX-2 enzymes and inflammation. Salicylates are absorbed 3-4mm below the skins surface. There is little clinical data supporting the effectiveness of salicylates in treating knee OA pain. Salicylates were not effective when applied directly to the affected knee joint in the only study, published four decades ago.8
The benefits to these products is that they have few side effects, however they should be discontinued if rash or skin irritation occur.1
Methyl Salicylate is the active ingredient found in OTC creams such as Bengay and Icy Hot. It too is a counter-irritant. On rare occasions it has been linked to severe toxicity and with topical application or accidental ingestion. It can also increase the risk of bleeding if used in conjunction with certain blood thinners.8
Menthol is the substance used in the Biofreeze. In one study it was demonstrated that menthol was comparable to ice in decreasing blood flow, and the 2 together demonstrated a potentiating effect, reducing radial blood flow by 39% at 10 minutes after application. The implication is that post-injury edema and swelling may be effectively controlled in this manner.
Capsaicin is the component of chili’s that makes them hot. It is a counter-irritant, which depletes substance P and gene-related peptide thereby desensitizing nerve fibers.8 Capsaicin may reduce the amount of neurotransmitters, which produce inflammation and alert the brain of painful impulses. Its clinical use is limited by the need for a sensitization period. Application of the crème causes a burning sensation, which after repeated use causes a pharmacological desensitization, and then a functional desensitization (analgesia). Most patients aren’t able to tolerate the burning and do not reach the analgesia phase.9
Capsaicin is not associated with any extreme adverse events. The most common negative effect is eye irritation if patients do not wash their hands after use and then touch their eyes.8
A 12 week double blind study on the efficacy of capsaicin 0.025% cream vs. a placebo was conducted on patients with knee OA. One hundred thirteen patients were included in the study and treatment was applied four times daily. Results showed that on the visual analogue scale patients decreased their subjective levels of pain by 54.1% in the capsaicin group compared with 38.3% in the placebo group. Also the physician’s global rating noted 81% of the capsaicin group and 54% of the placebo group as improved. It should be noted however, that the capsaicin group did not show superior decreases in pain on the VAS until the 4th week of usage. These researchers stated that capsaicin cream was only somewhat effective, providing mild analgesia in certain patients. It was recommended that capsaicin be used as an adjunct to other treatments as opposed to the sole treatment.8