I didn't find a thing on the internet about melatonin causing diarrhea.  I dropped my dose to 3mg and didn't really have any problems.  I got really constipated again and took 5 mg.  It's not causing diarrhea now, but it makes me "go".  So I routinely take it again and I'm staying fairly regular.  If you find anything, I'd love to hear about it.

I did find out that it can cause "abdominal discomfort" and one side effect is diarhea. I think I'll stay off it for a while and then go back on with a very low dose. I included what I found but if you were doing research on it you probably already know most of it. They think it may prevent some kinds of cancer. I did find that another side effect can be itching. I've had a bad problem with that and we think caused by fibromyalgia, so I thought I would let you know that in case you should start itching you might want to discontinue the melatonin and see if that helps.

Happy holidays,

Food Sources: Oats, sweet corn, rice, ginger, tomatoes, bananas, barley, Japanese radish. Tryptophan, which can be converted by the body into melatonin, is found principally in spirulina, soy products, cottage cheese, chicken liver, pumpkin seeds, turkey, and chicken.

Effects: Regulates the body's clock, so it can be effectively used to treat insomnia, jet lag, and depression. Unlike sleeping pills, it doesn't interfere with REM sleep (the dream state), and leave the individual feeling groggy upon wakening, and it resets the body's clock rather than just knocking the person out. It is known to produce vivid dreams. It also works as a powerful antioxidant (as much as 500 times more powerful than DMSO), and is now being tested as a possible anticancer drug (it may inhibit some types of cancers, particularly those that are hormone-related - e.g., breast cancer, prostate cancer - when taken in doses of 40 mg a day or more). It has also been shown to boost the immune system, lower cholesterol, shrink the prostate, and lower heart rate and blood pressure. Researchers are looking into it as a possible treatment or cure for such things as Alzheimer's, Parkinson's disease, AIDS, PMS, menopause, osteoporosis, Down's syndrome, and even as a birth control pill. Some suggest it may even delay the aging process, though it may not be able to reverse it (it has been shown to increase the longevity of lab rats an average of 20 percent), and it may ease some symptoms of autism, epilepsy and diabetes.

Taking adequate amounts of vitamin B-3 can free up more tryptophan to be converted to melatonin. Vitamin B-6 can stimulate the body to produce more melatonin, as the body uses it to convert tryptophan into serotonin, which can then be converted to melatonin. Taking calcium and magnesium at night may also benefit melatonin production.

Precautions: It should not be taken by women who are pregnant, nursing, or trying to get pregnant, as it interferes with sex hormones; healthy children, as they produce enough on their own; those with severe mental illness, as it could worsen the symptoms; those taking steroid drugs such as cortisone and dexamethasone, as it could reduce their effectiveness; and those with overactive immune system abnormalities, including lymphoma, leukemia, allergies, and autoimmune diseases such as arthritis, multiple sclerosis, and lupus, as it could worsen these conditions.

It appears to be nontoxic, though much research still needs to be done, and harmful side effects may yet be uncovered. Most studies have been done on rats, which do not produce their own melatonin. Side effects may include depression, diarrhea, headaches, insomnia, nightmares, grogginess or fuzzy thinking upon arising from sleep, and a reduced sex drive. Taking it at the wrong time could cause problems, such as falling asleep at the wheel of a car; taking too much (more than 0.3 mg) in efforts to fall asleep could result in a melatonin hangover the next day. Paradoxically, women who take it for birth control do not get sleepy; why this happens is not known. When taken counter to the body's normal circadian rhythm (e.g., by those working night shifts), it can have negative effects on memory and cognitive processing. Optimum dosage has not yet been determined, and long-term effects are still not known. There may be a risk of contamination from viruses in the natural form, made from animals, as opposed to the synthetic form; on the other hand, most synthetic forms are manufactured outside the U.S., leaving open the question of quality control. Apart from this, natural and synthetic forms are identical. Melatonin is only effective as a sedative if the pineal gland's production is low; otherwise, doses as high as 75 to 150 mg will have no effect. It should not be used to treat insomnia, as there are numerous causes that result in a loss of sleep, and melatonin may not be effective in countering them. One study has shown that it can worsen clinical depression in some cases, and in nondepression cases can cause fatigue, confusion, and sleepiness.

Some medications, especially hypertension medicines, can interfere or react with melatonin. The production of melatonin can also be inhibited by exposure to electromagnetic fields, lack of sunlight during waking hours, lack of darkness during sleep (caused even by such seemingly innocuous offenders as night lights), aspirin (which can cut production by 75 percent), alcohol, beta-blockers, calcium-channel blockers, sedatives (such as Prozac, Valium, and Xanax), steroids, fluvoxamine (Luvox), desipramine (Norpramin, Pertofrane), most MAO inhibitors, caffeine (including coffee, green tea, colas, and chocolate), tobacco, doses of B-12 above 3000 meg, ibuprofen, sleeping pills, tranquilizers, and indomethacin (which can completely block the nighttime increase of melatonin). Smoking one joint of marijuana, on the other hand, can increase melatonin levels in the body 4000 percent.

The sale of melatonin is banned in Canada, France, and Britain.

Dosage: Levels produced by the body are measured in picograms (trillionths of a gram), averaging around 10 pg per milliliter of blood per day and 50 to 150 pg per milliliter at night. Melatonin output in the body can be improved by eating the foods mentioned above; getting enough niacin, B-6, calcium, and magnesium; meditation, prayer, and other relaxation techniques; and smoking marijuana. Sublingual supplements allow the melatonin to reach the brain quicker, but a time-release supplement at bedtime is probably best, as it most mimics natural conditions. Researchers recommend taking only 0.5 to 5 mg a day one-half to one hour before bedtime, though some studies have shown that as little as 0.1 mg (equal in amount to a few grains of salt) can enhance sleep. Most pills are 2.5 to 3 mg, which, according to Dr. Russel J. Reiter, professor of neuroendocrinology at the University of Texas Health Science Center in San Antonio, may be "overkill," though probably not harmful. Women taking up to 75 mg a day for birth control research have shown no apparent side effects. Reiter himself takes less than 1 mg/day, and Ray Sahelian, M.D., does not recommend its use more than once or twice a week. On the other hand, Crook and Adderly advise its use only for combating jet lag, and Alan Gaby, M.D., who publishes the newsletter Nutrition and Healing, states, "We do not at this time believe that melatonin should be a routine part of the average nutritional-supplement program." Reiter recommends an alternative program of 100 mg of B-3, 1000 mg of calcium, and 500 mg of magnesium at night, plus 25 to 50 mg of B-6 early in the morning, to assist the body's own production of melatonin.

I have been using Melatonin 10mg. for almost a month, and have been experiencing watery diarrhea and itching severly for that amount of time.  My doctor has been cutting down, or cutting out a few of my other medications to rid me of both of these horrible side effects.  Nothing changed.  I never suspected it was the Melatonin.  Today, for some reason, I thought to look Melatonin up on my computer.  I was shocked to say the least.  I have been reduced to wearing diapers because there have been too many times that I have not been able to get to the bathroom, and had to throw away my underwear and some clothes.  My action has been, as of this day (July 12, 2011), to stop taking Melatonin forever.  I will let you know the results.

I'm glad you figured it out about the melatonin.  I started taking it again, but I only use 3 mg.  I ran out and didn't take any for awhile and I really didn't notice it.  Why I bought more is beyond me.  I'm hoping to hear that the diarrhea is gone.

I'm so glad every one posted their experiences. My husband and I are caring for his elderly mom who has alzheimers and has alot of difficulty sleeping. We started her on Melatonin at the suggestion of the neurologist, and it the beginning it seemed to be working. After about a month we were up to 12mg a she was having daily episodes of stool incontinence. She also was reduced to wearing diapers. I brought her to the Doctor had stool samples and blood work drawn, we were even considering placing her in a nursing home,... it was that bad. I can't thank all of you for sharing. We stopped the Melatonin and life in our house is so much better.

I started taking  3 mg Melatonin about 5 weeks ago. I began having diarrhea approximately the same time. I have about 4 to 5 bouts a day and then am good the rest ofthe day. I have had problems in the past with diarrhea but not this bad. At that time the Dr changed my cholesterol medication from simvastatin to another and the diarrhea stopped. The new medication was not generic so I decided to change back. The diarrhea didn't return until  taking Melatonin. There are other nasty side affects if you have high blood pressure or other medical problems. I have noticed my heart skipping beats more than usual as well. I found these side effects by googling "Melatonin cause diarrhea?". Thanks so much for the posts about experiences with this product.

Mayo Clinic does not recommend more than 6 mg, but I had to stop d/t severe diarrhea

UPDATE:

I am further informed that I can only trust my personal line of sight knowledge absent which my son eats bananas hidded in the frig peel and all, grapefruit

bitten, and sweet green pepper since he is allowed by his Mom to roam and do

as he pleases.

Experience means everything.  Years ago I was taking a medication that caused some side effects that were nasty.  A physician told me that the PDR said nothing about those side effects and therefore did not cause my problem.  Well, I stopped and they went away.  Tried it a few times....like you.  About 2 years later the PDR now lists that side effect for this medication.  So now I trust myself and my experience. 

This side effect was mentioned on TV?  It's about time!  I'm so sorry the melatonin caused so many problems for you.  At least, you now know the cause of the diarrhea.