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Considering her age, you might want to give her a lower dose such as 1/4-1/2 of normal adult dose and see if burning goes away.
Or we can consider other laxatives that will help her such as Colace, Glyerin suppositories, Senna.....that are milder version for elderly.
Morphine can make constipation worse too. That is why patients are given Colace to use along with these opiate pain killers.
Glycerin is an osmotic laxative as wll. Psyllium seed preparations is another good option.