This article illustrates blood brain permeability of PYRIDOstigmine:

http://www.nature.com/nm/journal/v2/...1296-1382.html

Normally this drug does not cross the BBB...but people do have issues, which may allow some drugs and other substances to cross. The channels in the BBB are called Zonulin channels and are also in the GI tract. They are little holes, which open and close.

Older patients tend to have more permeable BBB's. Some drugs that do not cross when younger will cross in the elderly giving central side effects. The H2 antagonists are one class that do this. Zantac and Pepcid. It is not unusual that elderly on IV Tagamet/or Zantac hallucinate on these drugs.

This article states that neostigmine also does not cross.
http://en.wikipedia.org/wiki/Neostigmine

There have been many many discussions on Parkinson's here about the blood brain barrier and permeability. If your do a search there you will find some very interesting articles with scientific citations.