There are a couple differences between medications that are worth noting; meperidine or
Demerol should be avoided in the doses you are describing, as the primary breakdown product, normeperidine, is neurotoxic and causes seizures; Codeine is not a good choice at these doses as it just does not have that much potency; Morphine does tend to cause more stomach cramping and nausea, and is subject to high 'first pass metabolism' that makes oral dosing difficult; Any opiate at these dos