For new drugs, efficacy is shown by randomised controlled trials to enable their registration, then further studies and clinical guide the actual practical application and subtleties in the implementation of that new therapy and, ultimately, its success or failure. The same principles apply to dietary therapies. Obtaining high-quality evidence of efficacy has far greater challenges than with drugs because of the limitations in study design, including problems with placebos and blinding. Nevertheless, there are now several comparative and randomised controlled trials of varying design providing convincing evidence that the low FODMAP diet has efficacy in IBS. However, demonstrating efficacy alone is insufficient to lead to the diet being rolled out as a first-line therapy. Multiple issues need to be addressed. Questions might include the following: Does it work in the real world? Does the diet need to be delivered by a dietitian or can it be successfully self-taught? How does the diet compare with other diets that are believed to be of benefit? Is nutritional adequacy achieved when implemented in the community? What is the durability of the diet? Is it safe? How difficult is it to learn? Some of these issues have been scientifically addressed, but many have not. These issues will be discussed to provide the basis for making one’s own call as to whether the low FODMAP diet is ready for primetime as a first line therapy.