Purpose: Diets other than those recommended by professionals, referred to in the present paper as "pseudo-diets", can endanger the health of the people who follow them. It is therefore worth assessing why people begin such diets, the information they rely on, and the effects they experience. Materials and Methods: We surveyed and compared people following two pseudo-diets: a ketogenic-like diet (KLD) and a vegan-like diet (VLD). The diets are defined as ketogenic and vegan by the dieters themselves. A cross-sectional study was carried out using self-developed anonymous online questionnaires. The survey participants were adults: 249 KLD and 203 VLD followers. Results: The majority (85.14%) of the KLD followers stated that their motivation was weight loss, while 56.16% of the VLD followers stated that they were primarily motivated by ethical considerations. Only 11.64% of the KLD followers and 33.99% of the VLD followers had sought professional help. Both the variety and frequency of the adverse effects were robust in the KLD group, while the VLD followers experienced primarily positive outcomes. We found a statistically significant association between the seeking of professional help and an increase in desirable effects in both groups, and a decrease in adverse effects in the VLD group. Conclusions: The dieters used several information sources but only occasionally turned to professionals. Given that dietary changes can represent a significant intervention, professional monitoring is highly recommended to ensure that the diet is valid, effective, personalised and safe.  Kulcsszavak: pseudo-diet, ketogenic-like diet, vegan-like diet, desirable and adverse effects