The insoluble compounds precipitate out of the water, while the soluble ones stay dissolved.
[1][2] It is used to separate out opiate drugs that have been mixed with common non-opiate analgesics.
When cold water extraction is used with codeine/paracetamol, hydrocodone/paracetamol and oxycodone/paracetamol medications, it is not effective at removing all of the paracetamol.
[3] The cold water extraction process is fairly simple not only due to the simplicity of the method but also because the items needed are minimal and can all be found in a common household.
Extraction of controlled substances from over-the-counter or prescription drug formulations may be illegal in some countries.