The Proto-Indo-European root is *ḱsero- "dry". This became Greek ξηρός ksēros "dry, arid" and ksērion "desiccative powder for wounds". ksērion was borrowed into Arabic as al-'iksīr (أَلْإِكْسِير ?) (with an epenthetic initial vowel and the definite article al), and generalized to mean "elixir". This was borrowed into medieval Latin as elixir.
The suffixed variant *ḱseres-no- (*-no- formed adjectives) became Latin serēnus "serene, bright, clear", borrowed into English as serene.