I would wonder if the oral forms are exerting a local effect on the GI tract while they are being absorbed?
Injection also by passes the GI tract and liver, allowing for no first pass metabolism.
Neostigmine is used to reverse ileus of the bowel, so it does depending on dose affect motility. It may be the injection you are using is a low dose, compared to others.
http://www.rxlist.com/prostigmin-drug.htm
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Neostigmine bromide is poorly absorbed from the gastrointestinal tract following oral administration. As a rule, 15 mg of neostigmine bromide orally is equivalent to 0.5 mg of neostigmine methylsulfate parenterally, due to poor absorption of the tablet from the intestinal tract. In a study in fasting myasthenic patients, the extent of absorption was estimated to be 1 to 2 percent of the ingested 30 mg single oral dose. Peak concentrations in plasma occurred 1 to 2 hours following drug ingestion, with considerable individual variations. The half-life ranged from 42 to 60 minutes with a mean half-life of 52 minutes.
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I could not find an exact statement about absorption for PYRIDOstigme (Mestinon), but I did find this paper, which suggests pyridostigmine is eliminated a bit slower than neostigmine.
http://www.ncbi.nlm.nih.gov/pubmed/8845827
None of these agents have a high profile of GI absorption though.
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Weezie looking at petunias 8.25.2017
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