Cold water extraction
Cold water extraction is the process whereby a substance is extracted from a mixture via cold water. It is a type of fractional crystallization.
The process generally involves taking a mixture of substances, and dissolving them into warm water, and then cooling the mixture. The insoluble compounds precipitate out of the water, while the soluble ones stay dissolved. The solution can then be separated through filtration or decantation.
This process works by exploiting the differences in solubility (with respect to temperature) of varying substances.
Opioid extraction
In some countries tablets are available over the counter (in others these are available only by medical prescription) that contain aspirin, paracetamol (acetaminophen) or ibuprofen in combination with codeine, an opiate. Cold water extraction of codeine has become popular amongst recreational drug users because it speeds up absorption and reduces hepatotoxicity caused by paracetamol and gastrointestinal irritation caused by aspirin and ibuprofen. However, without careful measurement of the insoluble product removed during the process, it is conceivable that one could actually cause damage to the liver by using many low dose tablets in an inefficient process. This method is also used to extract hydrocodone and oxycodone from similar (usually prescription-only) compounds.
These extractions are possible because opioid salts (codeine phosphate [1], hydrocodone bitartrate [2], oxycodone hydrochloride [3]) have a higher solubility in water at lower temperatures than paracetamol [4] and ibuprofen [5]. The more water cools, the less paracetamol and ibuprofen the solution will retain in proportion to the opioid salts.