Hello, Meclizine is antihistamine which is used to treat vertigo. Constipation is very less likely to be due to meclizine. There must be something else most importantly dietary habits are leading to constipation. I would advice you to make sure you include food with lots of fibers in it, which helps in relieving constipation. Color of the stool is likely related to some other cause as its seen with different illnesses. Even after this there is no relief then see your doctor. With all history and doing a physical examination and analyzing your situation, your doctor will suggest you stool softner syrups which will help in the problem of constipation. Apart from this if there is need of any blood work or other tests, then you doctor will guide you on that. Hope I have answered your query. Let me know if I can assist you further. Regards, Dr. Manisha Auradkar, General & Family Physician