This is Dr Basu, Experienced Specialist.
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That can be frustrating.
First of all, she needs a colonoscopy to rule out any rectal tumor which can cause this sensation.
If normal colonoscopy, then this is a nerve related problem.
MS can affect rectal nerves also.
In that case, Amitriptyline 25 to 50 mg at bedtime can be used daily.Loperamide
tablets also can be used as needed.
in fact loperamide is over the counter.
Another medication that can help is an anticholinergic one like oxybutynine or hyosciamine.
Please discuss with a GI specialist.
Thank you for sharing your concerns with us today. If you have any additional questions or need further clarifications, please do not hesitate to ask. I am here to answer them. I wish you good health. Regards, ***** *****