Diarrhea or dysentery is often caused by food poisoning or exposure to contaminated water. After TEOTWAWKI, this would become more common due to lack of refrigeration, clean water and good hygiene.
In the old Special Forces Medical handbook, there is an appendix, called Primitive Medicine in a Survival Situation that discusses this issue. This was written by a doctor that spent time in a North Korean Prisoner of War Camp where they had a real problem with severe dysentery. They set the standard for dysentery at 25 stools per day, 8 to 10 was considered normal and 15 just simple diarrhea.
Since they had no modern medications they had to treat it with improvised meds. To treat it, he stated that they used charcoal scraped off a piece of burned wood. They used about a handful for dysentery and the patient would have to swallow it. They also burned and ground bones in ash and swallowed the powder. Schoolroom chalk was powdered and swallowed.
They also took bark for oak trees and would boil it in water for between 12 hours to 3 days. The resulting balk concoction was vile tasting and hard to get down, but worked due to it containing tannic acid. If addition to the above treatment they also encourage the drinking of large amounts of boiled water for rehydration.
Activated charcoal U.S.P, is a pure naturally produced, wood charcoal carbon. One teaspoonful has a surface area of more than 10,000 square feet. This allows it to adsorb large amounts of chemicals or poisons. It works much better than the raw charcoal they had in the POW camps. Because of its ability to absorb contaminates always store charcoal in a tightly sealed container.
Activated charcoal U.S.P. can be purchased at most pharmacies and health foods stores. I have also heard of people buying activated charcoal at pet stores. It is used in the filters for aquariums. If you use the fish type, you may need to grind it into finer pieces. If the diarrhea is caused by food poisoning, a half a teaspoon or so will often bring quick relief. It also works great for the relief of heartburn. I often use it for mine.
Warning: Briquettes for grilling food are not safe sources for either external or internal use, as various fillers and chemicals are applied to hold the charcoal together and to insure rapid igniting.
Activated charcoal is not for everyday use as it adsorbs important vitamins and minerals you need to be healthy. You should wait 2 hours before taking any other kind of medication or meds may be neutralized.
I want you to be sure and understand that I am not a Doctor and have not had any specialized training. You need to verify this information with your own doctor prior to any attempt to treat yourself based on information in this article. This for information only.