The only thing that stands out from her medication is that Zanaflex can be associated with urinary frequency (probably due to the effect on smooth muscle). Other obvious causes of nocturia should also be excluded such as bladder prolapse and diabetes, so she needs to have her blood and urine sugar level checked.
Interstitial cystitis also needs to be ruled out. If the urine is very concentrated then this can sometimes cause frequent emptying of small volumes so increasing her fluid levels early in the day (but reducing after 6pm) may help. She should cut out alcohol and coffee as these can worsen the symptoms too.
If all else fails then a catheter with night bag may be the solution to allow her to return to a normal sleep routine,. It may be worth trying for a couple of weeks and then stopping to see if that has helped 'reset' bladder emptying habit.
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