What is best medication for constipation..90 years old?

Need more than that: 90 year old will need not just medications , ( any over the outer medication is good ) have to find the cause , make sure no colon cancer , if home bound visit by health care worker to make sure no fecal impaction the most common problem in elderly.

Osmolar laxatives: If fiber, hydration and activity has failed, the preferred class of laxatives pull water into the large bowel. They are less likely to cause dependence and are well tolerated. Lactulose also changes the ph of the gut increasing motility leading to passage of stool. Avoiding stimulant laxatives is generally recomended (there are of course circumstances where they are necesary).