OTC Transdermal Analgesic Patches in Pain Management

Donna M. Lisi, PharmD, BCPS, BCPP, BCGP, BCACP

Disclosures

US Pharmacist. 2019;44(3):15-21. 

In This Article

Methyl Salicylate

MS is also referred to as oil of wintergreen, sweet birch oil, gaultheria oil, teaberry oil, and mountain tea.[14] Medicinally, MS has been used an analgesic, an anti-inflammatory agent, a rubefacient/counterirritant, a fragrance in health and beauty aids, and a penetration enhancer for other topical medications.[31] It possesses keratolytic, bacteriostatic, fungicidal, and photoprotective properties.[32] It has both vasodilatory and thermogenic action, increasing local blood flow and skin temperature, respectively. It is by this mechanism that MS acts as a rubefacient.[31]

A systematic review found that while topically applied rubefacients containing salicylates may be effective for acute pain (NNT 2.1 for at least 50% pain relief), counterirritants have moderate to poor efficacy for musculoskeletal and arthritic pain (NNT 5.3).[15] A Cochrane Database Systematic Review showed that that there is a lack of evidence to support the use of topical salicylate-containing rubefacients for the treatment of acute injuries (mostly sprains, strains, and acute low back pain) or chronic conditions (mostly osteoarthritis, bursitis, and chronic back pain).[33]

A small placebo-controlled trial (N = 30) showed that both 10% MS/3% l-menthol patches and 5% lidocaine patches are effective in relieving myofascial pain. Whereas the lidocaine patch was associated with a slightly better reduction in pain intensity and a small increase in the degree of mouth opening and quality of life, the MS/menthol patch was more beneficial in increasing lateral movement compared with its competitors.[34] However, this study was of poor quality; demographic data of the study population were not even provided.

OTC topical products may contain up to 20% MS. The drug is lipophilic. There is appreciable dermal absorption via direct local tissue penetration when MS is applied topically.[35–37]

Acute salicylate poisoning has occurred following dermal application of MS. Exercise and heat exposure (i.e., use of a heating pad, sauna, hot shower) increase percutaneous absorption. Absorption is also directly proportional to the amount of skin in contact with the drug. When coadministered, phenolic compounds such as camphor or menthol may damage the skin's protective capacity and enhance the skin penetration of MS. Topical exposure of salicylates has been associated with fatalities from salicylism, especially in children.[32,38]

MS demonstrates zero-order kinetics following dermal absorption.[31] Topical MS also inhibits both central and peripheral prostaglandin synthesis.[14] Similar to oral aspirin, topical MS can interact with oral warfarin, resulting in decreased platelet aggregation, prolongation of the international normalized ratio, and bleeding.[38,39] Because of its salicylate component, MS should not be given to children (over concern of Reye's syndrome) or those who are allergic to aspirin.[14]

Severe urticaria, angioedema, and contact dermatitis have been reported with the use of MS.[38,40] Full-thickness skin and muscle necrosis has been reported with the use of combination MS and menthol products and the concomitant application of a heating pad.[38]

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