4 doctors agreed: Maybe not allergic: It's not unusual for pain medications such as morphine, demerol, or Dilaudid to be associated with some itching. They all can cause histamine release. This doesn't necessarily mean you are allergic to them. True allergy produces severe symptoms such as hives and trouble breathing. Sometimes Benadryl can help with itching. Or you can ask your doctor to try a different pain medication.
What is the most important electrolyte for the maintenance of osmotic pressure and volume in the extracellular fluid compartment? The most abundant positive ion or cation in the extracellular fluid  is sodium (Na+). Chloride (Cl-) is the most abundant negative ion  or anion.
a chemical element, atomic number 11, atomic weight 22.990, symbol Na. (See Appendix 6.) Sodium is the major cation of the extracellular fluid, constituting 90 to 95 per cent of all cations in the blood plasma and interstitial fluid; it thus determines the osmolality of the extracellular fluid.