Why are nausea and vomiting reported as adverse effects of some parasympatholytics, such as oxybutynin, when nausea and vomiting are known to be parasympathomimetic adverse effects, and muscarinic antagonists can be used to treat nausea and vomiting? 

Parasympathomimetics often cause nausea and vomiting. Increased stimulation of muscarinic receptors in the gastrointestinal tract leads to increased secretion and motility, which can trigger nausea and vomiting.  Perhaps more importantly, activation of muscarinic cholinergic receptors, in particular, M1 receptors, in the vestibular nuclei, the nucleus of the solitary tract and the vomiting centre can directly activate the CNS pathways triggering nausea and vomiting. Hence, muscarinic receptor antagonists, such as scopolamine (hyoscine), can be used to prevent nausea and vomiting.

As muscarinic antagonists can be used to treat nausea and vomiting, it can be confusing that some parasympatholytics, such as oxybutynin, are reported to cause nausea and vomiting.  Oxybutynin is a muscarinic antagonist. It can be used to treat urinary incontinence. Historically it was also important for treatment of peptic ulcers although now it is rarely used for this indication as we have better drugs such as H2 receptor antagonists (for example, cimetidine) and proton pump inhibitors (PPIs)  (for example, omeprazole).  However, the fact that oxybutynin can be used to treat peptic ulcers reminds us that this drug blocks M1 receptors on the enterochromaffin-like cells in the stomach and prevents gastric acid secretion. The nonselective muscarinic antagonism of oxybutynin also blocks secretions and motility all along the gastrointestinal tract. These effects can lead to delayed gastric emptying.  The delayed gastric emptying leads to nausea and vomiting.