Side effects are usually the first place owners notice whether reishi for cats is a good match. The most common concerns are gastrointestinal—soft stool, vomiting, or a sudden change in appetite. Less commonly, mushroom-related compounds can affect the nervous system, and certain mushroom intoxication patterns in cats can include salivation, vomiting, and diarrhea (Herreria-Bustillo VJ, 2013). While reishi sold as a supplement is not the same as a random wild mushroom, the takeaway is simple: unusual signs deserve prompt attention, not a “wait and see” mindset.
Stop the supplement and contact your veterinarian if symptoms are persistent, severe, or paired with lethargy. If there’s any chance your cat accessed unknown mushrooms outdoors or in houseplants, treat it as a potential poisoning scenario; some mushrooms can cause serious toxicity affecting the liver and nervous system (Puschner B, 2018).