Microalgal food supplements are becoming increasingly popular due to their promising biological effects and high nutritional value, evidenced in in vitro, in vivo, and human studies. Some products of this kind have, however, raised controversies concerning their safety. At the same time, not much is known about the frequency of adverse events following the use of microalgal supplements, potential factors that may influence them, and general characteristics and behaviours of the consumer group. The present study aimed to fill this gap and surveyed a group of Polish consumers of microalgal products (n = 150) using an online questionnaire. As found, microalgal supplements (Spirulina, Chlorella, and Aphanizomenon) were popular in groups representing lacto-ovo-vegetarianism and veganism and were consumed predominantly for nutritional, immune-boosting, and detoxifying purposes. Their use was rarely discussed with specialists; the Internet constituted the most important source of information regarding these supplements. The most frequently self-reported health-beneficial effects of supplementation included the following: increased immunity, higher vitality, improved hair and skin quality, and better general well-being. Diarrhoea, nausea, abdominal pain, and skin rash were among the most often reported adverse events. Pre-existing medical conditions, namely renal failure and hypothyroidism, but not Hashimoto’s thyroiditis, were associated with increased occurrence of side effects. Those individuals who had consulted specialists as to the use of supplements reported adverse events significantly less often. A strikingly high frequency of side effects and very low consumer satisfaction were reported by a group of consumers supplementing Aphanizomenon-based products. In summary, the present study highlights that microalgal consumers may benefit from additional warnings of potential side effects and from consulting a qualified health specialist prior to use.