Can constipation cause black stools? i have back stools but are small shaped and sometimes i strain i also have upper ab pressure but i go everyday

No: Dark stools are usually caused by the ingestion of iron or pepto-bismol. Eating excessive amounts of red meat and green leafy vegetables can cause your stools to turn dark as well. Spinach can be one example of that. Dark tarry stools that are extremely foul smelling on the other hand would indicate a G.I. Bleed and would require emergency attention. Best wishes.

See your physician: Black stool can be an indication of GI bleeding. Make an appointment with your primary care physician for an evaluation.