We don't have synonyms for antalgesic yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are analgesic, analgesia and analgetic.
(n) :
(uncommon) Alternative form of analgesic. [(pharmacology) Any medicine, such as aspirin, that reduces pain, especially without inducing a loss of other sensation. (Contrast anesthetic.)]
(adj) :
(uncommon) Alternative form of analgesic. [(pharmacology) Of or relating to analgesia; anodyne.]
(n) :
(uncommon) Alternative form of analgesic. [(pharmacology) Any medicine, such as aspirin, that reduces pain, especially without inducing a loss of other sensation. (Contrast anesthetic.)]
(adj) :
(uncommon) Alternative form of analgesic. [(pharmacology) Of or relating to analgesia; anodyne.]