US Pharm. 2011;36(7):50-51.
Method of Preparation: Calculate the quantity of each ingredient for the amount to be prepared. Accurately weigh or measure each ingredient. Heat the polyethylene glycol (PEG) 4,000 to about 75°C. Add the mephenesin powder and stir until dissolved. Add the capsicum oleoresin and stir until dissolved. Add the PEG 400, mix well, and cool to about 60°C. In a separate vessel, mix the methyl salicylate, menthol, camphor, and methyl nicotinate and stir until dissolved. Add this solution to the PEG mixture and mix well. Cool to room temperature while mixing. Package and label.
Use: This preparation has been used in the treatment of muscle pain and arthritis.
Packaging: Package in tight, light-resistant containers.
Labeling: For external use only. Keep out of the reach of children. Discard after ____ [time period].
Stability: A beyond-use date of up to 6 months may be used for this preparation.1
Quality Control: Quality-control assessment can include theoretical weight compared with actual weight, pH, specific gravity, active drug assay, color, texture–surface, texture–spatula spread, appearance, feel, rheologic properties, and physical observations.2
Discussion: Muscle and skeletal pain are often treated by combination therapy. This preparation contains six active ingredients that have various mechanisms of relieving pain.
Mephenesin (Tolserol, C10H14O3, MW 182.22) occurs as white, odorless or almost odorless, crystals or crystalline aggregates with a slightly bitter, numbing taste. It is soluble 1 in 100 in water and 1 in 8 of alcohol. It has been used topically, usually with rubefacients.3
Methyl salicylate (C8H8O3, MW 152.15) occurs as a colorless, yellowish, or reddish liquid with the characteristic odor and taste of wintergreen. It is slightly soluble in water and is soluble in alcohol and glacial acetic acid. Methyl salicylate should be preserved in tight containers; certain plastic containers (polystyrene) are unsuitable for liniments or ointments containing methyl salicylate. Methyl salicylate is absorbed through the skin and is applied topically in rheumatic conditions.2,3
Menthol (C10H20O, MW 156.27) occurs as colorless, hexagonal crystals, usually needlelike or in fused masses, or as a crystalline powder. It has a pleasant, peppermintlike odor. Menthol is slightly soluble in water and highly soluble in alcohol. It is freely soluble in fixed and volatile oils.4
Camphor (C10H16O, MW 152.23) occurs as colorless or white crystals, granules, or crystalline masses or as colorless to white translucent, tough masses. It has a penetrating, characteristic odor and a pungent, aromatic taste. Camphor will slowly volatilize at room temperature. It is slightly soluble in water, highly soluble in alcohol, and soluble in fixed and volatile oils. Topically, camphor is used as a rubefacient and mild analgesic.1,3
Methyl nicotinate (C7H7NO2, MW 137.14) occurs as a white or almost white powder. It is highly soluble in water and alcohol. Methyl nicotinate should be protected from light.3
Capsicum oleoresin occurs as a dark red, oily liquid that is soluble in alcohol, acetone, ether, and volatile oils. It is soluble in fixed oils but is opalescent in appearance. Capsicum oleoresin is a powerful irritant and may produce an intense burning sensation if it comes in contact with the eyes or tender parts of the skin. Take care to protect the eyes and to prevent skin contact with the drug. Capsicum oleoresin is primarily used externally as a counterirritant. It should be preserved in tight containers and labeled to indicate that, if separation occurs, it should be warmed and mixed before use.1,3
PEG (Carbowax, polyoxyethylene glycol) is an addition polymer of ethylene oxide and water. At room temperature, PEGs with molecular weights of 200 to 600 are liquid and those with molecular weights greater than 1,000 are solid. Liquid PEGs are clear, colorless or slightly yellow-colored, viscous liquids with a slight but characteristic odor and a bitter, slightly burning taste. The freezing point of PEG 400 is 4°C to 8°C. Solid PEGs are white or off-white pastes or waxy flakes. The melting point of PEG 4,000 is 50°C to 58°C. All PEGs are soluble in water and miscible in all ratios with other PEGs. PEGs are chemically stable, do not support microbial growth, and do not become rancid.5
REFERENCES
1. USP Pharmacists’ Pharmacopeia. 2nd ed. Rockville, MD: US Pharmacopeial Convention Inc; 2008:775-779,1428,1448.
2. Allen LV Jr. Standard operating procedure for performing physical quality assessment of ointments/creams/gels. IJPC. 1998;2:308-309.
3. Sweetman SC, ed. Martindale: The Complete Drug Reference. 37th ed. London, England: Pharmaceutical Press; 2011:89,2061,2489,2492.
4. Langdon BA, Mullarney MP. Menthol. In: Rowe RC, Sheskey PJ, Quinn ME, eds. Handbook of Pharmaceutical Excipients. 6th ed. London, England: Pharmaceutical Press; 2009:433-435.
5. Wallick D. Polyethylene glycol. In: Rowe RC, Sheskey PJ, Quinn ME, eds. Handbook of Pharmaceutical Excipients. 6th ed. London, England: Pharmaceutical Press; 2009:517-522.
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