A recent paper on case studies observed in the Princess Margaret Hospital, Hong Kong, begs to differ. Over the period between 2004 and 2009 sixty-six cases were observed (one of which was fatal) and eighty-one products were analysed and found to be adulterated, many with multiple contaminants.
Sibutramine was the most commonly encountered illicit agent in the current study. Consistent with previous reports [4, 7], cardiovascular effects were frequently observed. [...] Phenolphthalein, a laxative that was withdrawn due to potential carcinogenicity, was found in illicit weight-loss supplements both in the current as well as previous studies
. Similarly, despite being banned, fenfluramine has been frequently encountered as adulterants. [...] Laxatives and diuretics were found in illicit proprietary weight-loss products, and are presumed to decrease body weight by loss of water and bowel contents. However, these compounds do not possess genuine or long-lasting weight-loss effect. Animal thyroid tissues were also detected, as has been reported previously.
As Science 2.0 notes, this is of concern not just for Hong Kong residents:
While the research concentrated on cases in Hong Kong, the work raises worldwide concerns. These slimming products are widely available over the counter not only in Hong Kong, but in other countries where drug regulation is relatively non-comprehensive. In addition, anyone can buy them over the Internet even if they live in regions with tighter regulatory control.
So it would seem those people seeking out the 'natural' alternative to avoid all those dangerous drugs and their nasty side-effects are often doing neither and, in fact, often utilising dangerous and withdrawn drugs to achieve their goals.
Tang, M.H.Y., Chen, S.P.L., Ng, S.W., Chan, A.Y.W., & Mak, T.W.L. (2010). Case series on a diversity of illicit weight-reducing agents: from the well known to the unexpected. British Journal of Clinical Pharmacology. “Accepted Article”; doi: 10.1111/1365-2125.2010.03822.x