A question from my email:
"After I went to the bathroom today I was surprised to discover my stool was green! What can cause this? Should I be worried? Should I call the doctor?"
If you have the occasional green stool—don't panic! Most likely it is normal, and there is a perfectly reasonable explanation. Think about the foods you ate, any vitamins or supplements you took—even if they're not green in color.
There can be many reasons why stool appears to be green. The first and most obvious reason is eating green foods. Green, leafy vegetables contain chlorophyll which could be coloring the stool green. Foods with dark purple coloring like Kool-Aid, popsicles, and gelatin (Jell-O) can also result in rainbow-colored stools. Iron supplements or even foods that are rich in iron can also give stool a green tinge.
Stool can also appear green for physical reasons, and not just from what you've been eating.
We think of a healthy stool as being brown. Bile that is secreted in the first part of the small intestine is actually green. As stool is digested and passed through the large intestine it is turned into a darker brown color.
If stool is still green by the time it is excreted, it could mean that it went through the large intestine too fast to be changed in color. This is often called "rapid transit" or "decreased colonic transit time" and diarrhea that is green in color could be the result.
In breast-fed babies, green stool is a normal occurrence, especially right after delivery. In older children, the reason could be food-related as described above, or even from non-food items. Eating or sucking on colored non-food items such as crayons might also turn stools different colors.
If green stool continues, or your bowel habits have changed in other ways, you may want to call your doctor. This is especially true if you are experiencing diarrhea which can be an indication of other problems or could lead to dehydration.

Q. What are the causes of mucus in the stool?

Stool normally contains a small amount of mucus, but passing stools with visible amounts of mucus can be from a variety of causes. Mucus in the stool is a common symptom of irritable bowel syndrome (IBS) and ulcerative colitis (UC), and is seen to a lesser degree in Crohn's disease (CD). A bacterial infection, anal fissure, or a bowel obstruction may also cause the passage of mucus.
A.
Mucus is a clear, white, or yellow substance with the consistency of jelly which is produced by the mucous membrane of the large intestine. Mucus is also produced by other organs in the body such as the lungs, where it helps to trap any foreign particles that are inhaled. In the intestine, mucus protects the inner lining and helps ease the passage of stool.

Ulcerative Colitis and Irritable Bowel Syndrome

In UC, the mucus membrane of the intestine becomes inflamed and develops ulcers. These ulcers bleed and produce pus and mucus, which may be visible as they are passed in the stool. In IBS, there may be increased mucus production by the lining of the intestine, which is then passed. Mucus is more often associated with diarrhea-predominant IBS. Mucus is less frequent with CD, but may be associated with the development of an anal fissure.

Bacterial Infections

Bacterial infections, such as those from Campylobacter, Salmonella, Shigella, and Yersinia may also cause mucus in the stool.
A bacterial infection may also cause diarrhea, fever, and abdominal cramps. Some bacterial infections may resolve on their own without treatment, but some cases may be serious and require treatment with antibiotics.

Bowel Obstruction

A bowel obstruction is associated with constipation, severe cramps, abdominal distention, and vomiting as well as the passage of mucus. Bowel obstructions can be from a variety of causes and is typically treated in the hospital, with surgery being necessary in some cases.
The passage of mucus in the stool in the setting of IBS or UC is not necessarily a cause for alarm, but should be mentioned to a physician. Mucus without an underlying cause such as a pre-existing condition should be reported to a physician immediately.