SECRETIN
Secretin is a polypeptide hormone involved in the regulation of gastric function. Following administration by intravenous injection, it causes an increase in the secretion of the pancreas of water and bicarbonate into the duodenum. It is used alone or in conjunction with pancreozymin or cholecystokinetic agents as a test for exocrine pancreatic function, and in the diagnosis of biliary-tract disorders.
Secretin has been known for at least 20 years and it has a variety of functions but not much in the way of medical uses. One thing it has been used for is to test pancreatic function in other words to see if the pancreas is functioning correctly. A small amount is injected and the amount of "bicarbonate" which appears in the bloodstream is measured a short time afterwards. The bicarbonate secretion is required in order to neutralise the acid from the stomach and allow the enzymes in the duodenum to function. As well as secreting bicarbonate, the pancreas secretes many other enzymes including lipases and especially peptidases. These peptidases will break down the peptides, which, according to proponents of the opioid excess theories of autism, may be responsible for the problems. One way to diminish the problems caused by these potentially harmful peptides is to remove them from the diet. That is why people experiment with gluten and casein free diets. Since secretin will stimulate the pancreas to produce these enzymes it could ameliorate the symptoms by this mechanism.
It could, also or alternatively, be acting in the brain itself or in the intestinal wall (if it acts at all). The drug without side effects does not exist. Secretin has not been used over periods of time so we don't know what will happen when it is. With products as complex as this we should expect them to occur. The benefits/advantages ratio will have to be taken into account. There are some children with whom no effects are seen anyway.
Here is a link to how the BBC broke a story of an American lady who found success with this treatment.