Codeine: general

Codeine is an opiate used to treat mild to moderate pain and to suppress cough [1][2]. It's a natural alkaloid found in opium, making up a small percentage of the raw material [1], and is also produced by methylating morphine [3].

Codeine is a prodrug — your body has to convert it into morphine before it works, using a liver enzyme called CYP2D6 [1][4]. How much pain relief you get depends heavily on how active that enzyme is in you, and that activity varies genetically from person to person [5]. You likely don't know which kind of metabolizer you are.

Serious or life-threatening breathing problems are a known risk, particularly in the first day to three days of use [2].

Low-dose codeine (8 mg or less per tablet, 20 mg or less per 30 mL) is available from pharmacists without a prescription in some places [4].

Used at high doses over a long period, codeine is addictive [4].

References

1. Codeine | C18H21NO3 | CID 5284371 — National Library of Medicine (PubChem)
2. Codeine: MedlinePlus Drug Information — National Library of Medicine (MedlinePlus)
3. Codeine (PIM 140) — IPCS INCHEM (World Health Organization)
4. Codeine: Uses, effects and risks — Government of Canada
5. Codeine - StatPearls — NCBI Bookshelf (StatPearls Publishing)