Background & aims: HMG-Co reductase inhibitors (e.g. simvastatin) are important coronary heart disease (CHD) risk-reduction agents. Prescription protocols recommend low-fat diets in conjunction with simvastatin prescription. Simvastatin efficacy studies were reviewed to define dietary contexts within which simvastatin is most efficacious. Methods: Randomised controlled trials of simvastatin were identified by a literature search of Medline Databases (1992-2006), English-language journals, and reference lists of original articles, reviews and metanalyses. Studies involving both genders, sample size >30, were assessed for published details on dietary advice, dietary intervention duration, dietary adherence measures, and dietary data analysis. Results: Review of 18 selected studies revealed an absence of transparency of dietary control frameworks and analysis. Even where a specific diet was integral to study design, information was scant on diet-related outcomes or methods applied to assure dietary adherence. Conclusions: The absence of published details on dietary methodology reveals substantial gaps in understanding the context within which simvastatin is most efficacious. Further, these studies provide little scope for post hoc analysis for effects of specific dietary factors on simvastatin efficacy. Since dietary advice is often given in conjunction with prescription of many drugs, more rigorous attention to reporting dietary methodology is warranted.