Psyllium is used as an ingredient in bulk laxatives. Because of its high fiber content, it’s able to absorb water in the intestines. That makes the stool bulky and easier to pass. It’s important when treating constipation to drink plenty of fluids. This helps you avoid dehydration or a worse case of constipation. People allergic to English plantain pollen, grass pollen, or melon could have a serious allergic reaction when taking psyllium.