The main culprit seems to be the analgesic codeine phosphate - a controlled substance that is still freely available as part of other over-the-counter drugs, such as aspirin, paracetamol or ibuprofen.
Codeine phosphate is derived either from opium or morphine, and it's treated as a controlled substance in most countries, including the UK where it is available only by prescription.
It's not certain how many people are addicted to codeine-containing drugs, but one web service alone is helping 4,000 people who are addicted to Solpadeine, which combines codeine with paracetamol.
(Source: British Medical Journal, 2007; 334: 917-8).
E-news broadcast 10 May 2007 No.358 [Subscribe]