The rare side effects
Over the past decade, several case reports have described unusual, but serious side effects possibly attributable to the ketogenic diet. They include inflammation of the pancreas, prolonged QT intervals in heart rhythms, enlargement and problems with the heart muscle, selenium deficiency, severe carnitine deficiency, and basal ganglia changes (a deep part of the brain). We do not know at this time the true incidence of these problems, but they appear to be very rare.
The Charlie Foundation and many ketogenic diet experts are interested in creating a web-based side effect registry for the diet, as already exists for anticonvulsant drugs by the Food and Drug Administration. Physicians would enter anonymous information about a side effect they think may be due to the diet, and then could find out if any other doctors have seen it, too (and then contact them for how they treated the side effect.)
We are clearly in an exciting time for the ketogenic diet, but as its use grows, the occurrence of side effects will also grow. The future is bright because physicians are trying not only to identify side effects, but prevent them from happening.