What's the mechanism of action of Levomepromazine's analgesic effects?
According to Drugbank and the articles cited below, Levomepromazine is an antagonist of several different receptors: These include dopaminergic 2, dopaminergic 4, alpha-1 adrenoceptor, 5HT-2, histamine type 1 and muscarinic-cholinergic receptors.

References:


Evidence for the use of Levomepromazine for symptom control in the
palliative care setting: a systematic review
Muscarinic cholinergic and histamine H1 receptor binding of
phenothiazine drug metabolites
Levomepromazine receptor binding profile in human brain—implications
for treatment—resistant schizophrenia
Phenothiazine drug metabolites: Dopamine D2 receptor, α1-and
α2-adrenoceptor binding