Ketamine is an N-methyl-D-aspartate receptor antagonist; it also exhibits some binding to opioid ^ and О receptors. Morphine is a highly potent opiate analgesic drug; its pharmacologic action occurs through direct interaction with the ^ opioid receptors in the central nervous system. When used in conjunction with morphine, ketamine has a synergistic effect without causing an increase in the side effect profile.
Lau and others studied the effect of exposure to fluorescent light and pH adjustment on the stability of ketamine and morphine mixed together in normal saline. They reported that the combination was stable for 24 h when stored at 21°C with exposure to light. The mixture was also stable for 24 h when the pH was adjusted to 5.9 with sodium bicarbonate. Roy and Hildgren studied admixtures of ketamine and morphine diluted with normal saline and packaged in syringes, PVC bags, and medication cassette reservoirs. The 2 drugs, each at concentrations of 1, 10, or 25 mg/mL, were stable for 6 days when stored at room temperature. Schmid and others found that a mixture of ketamine (1.33 mg/mL) and morphine (2.0 mg/mL) was stable for 4 days at room temperature over the pH range
5.5 to 7.5.
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The use of ketamine in conjunction with morphine has become popular with the Acute Pain Service of the author’s institution. However, the short expiry date of 6 days with storage at room temperature does not allow for batch production without an increase in wastage. Therefore, a compatibility and stability study was undertaken to try to extend the current expiry date for ketamine (2 mg/mL) and morphine (2, 5, or 10 mg/mL) in normal saline in polypropylene syringes.