Here is a bit of nervous system trivia. Why so many of us have gut and digestive issues.
When the baby is in utero a mass of cells develops that begin the process of creating the brain and nervous system. After a few weeks these calls divide and one group becomes the brain and the other the GI tract. They come from identical cells.
Our 30-foot long gut is embedded with cells of the enteric nervous system, the ENS, a complex system of around 100 million nerves which is often referred to as our "second brain". The ENS supervises the processes of digestion and stays in close contact with, and is heavily influenced by, the central nervous system (the CNS) which comprises the brain and spinal cord.
The ENS and CNS use many of the same chemical messengers or neurotransmitters including acetylcholine and serotonin. In fact the gut has more of these receptors than the brain and can operate independently of the brain
When you take a drug that alters or boosts neurotransmitters in the brain it is also having an effect on the gut. Here is a web site that explains the process