Yes, it is a myth. The only time you will see anything like that is when someone is overdosing on iron salts or has a severe upper intestinal bleed.

By the way I cannot stand those Dual Action Cleanse commercials. They sit there talking about how harmful stimulant laxatives are since they weaken the intestinal muscles making you dependent on the laxative. This is all true. Then they claim their product is free of these stimulant laxatives. When they scroll the ingredients across the bottom of the screen the very first ingredient is cascara sagrada, a stimulant laxative. Check out their websites and try to find this out:

http://www.dualactioncleansenow.com/e-ingredients.aspx#colon


Notice how it is missing from the list of ingredients? Now read the fine print just below the list of ingredients. And if you click on the button to view the actual label you will again see cascara sagrada as the main ingredient. So why is it not listed in the main list of ingredients on the page?
Seems to me they are trying to hide this fact since they lied to the public claiming it was free of stimulant laxatives.

And here on their personal website they don't mention the ingredients at all:

http://www.dualactioncleanse.com/


When companies hide what is in their formulas that is a big red flag right there!