What reishi is, what the one dog study on it found, the safety notes that come from people, and how much reishi is in a daily chew.
Reishi mushroom for dogs shows up on a lot of chew labels lately, usually next to the words "immune support" and not much else. So if you've been reading the back of a jar in the pet aisle and asking yourself what this mushroom actually does for your dog, this one's for you. Below is what reishi is, what the one real dog study found, what's known about safety, and how much ends up in a daily chew. Short version... the dog research is thin but not zero, and most of the safety notes come from people.
Reishi is the common name for Ganoderma lucidum, a shiny, reddish-brown shelf mushroom that grows on hardwood. It's tough, woody and very bitter, so nobody slices it into a stir fry. It has a long history in traditional medicine in China, Japan and Korea, and today it's sold as capsules, tablets, powders, extracts and teas [2].
The parts researchers pay attention to are beta-glucans (a kind of complex sugar) and triterpenes, the bitter compounds [2]. A powder is the dried mushroom ground up. An extract is concentrated, so the same number of milligrams on two labels can mean very different amounts of those compounds. Keep that in mind any time you compare products.
Most of what you'll read online about reishi benefits for dogs comes from cell studies, mice and people. Turkey tail has a bit more dog research behind it, which we went through in what the research says about turkey tail mushroom for dogs. For reishi there's one controlled study in dogs, and it's a decent one.
Researchers gave 40 healthy adult beagles reishi powder sprinkled over their food once a day, at 0, 5, 10 or 15 mg per kg of body weight, for 28 days. On day 7 every dog got a rabies vaccine [1].
The dogs on the highest amount had white blood cells that were better at swallowing up particles in a lab test by day 28, and they made more antibodies to the rabies vaccine a week after the shot. Their food was digested just as well as the other dogs', and the researchers didn't see harm to their health at any of the three amounts [1].
What it doesn't show is just as useful. These were healthy research beagles, followed for four weeks. Nobody measured if they got sick less often, and a stronger vaccine response in a blood sample isn't the same as a dog who stays well. It's a real signal. It's one study.
People have more trials, and the results are mixed. Memorial Sloan Kettering's herb database lists modest benefits in some studies for urinary symptoms in men, blood sugar, cholesterol and anxiety, and no help for heart risk factors in type 2 diabetes or for fibromyalgia [2].
Your dog isn't a small person, though. Amounts tested in a 70 kg adult don't shrink neatly down to a 9 kg terrier, and an effect in people doesn't promise the same effect in her. So the human studies are a hint about where to look, nothing more.
In the beagle study nothing bad showed up at any of the amounts over four weeks [1]. That's reassuring, and it's also short.
The longer safety record comes from people. Reported side effects include nausea, trouble sleeping, dry mouth, constipation and dizziness. There are rare reports of liver injury too, including one fatal case linked to powdered reishi [2][3]. Reishi can also stack on top of some medicines: more bleeding risk with blood thinners, lower blood pressure with blood pressure drugs, lower blood sugar with diabetes medicine, and possible interference with drugs that calm the immune system [2][3].
So if your dog takes any of those, has a liver problem, or has surgery coming up, ask your vet before you add reishi. If she goes off her food or starts vomiting after you start it, stop and call. And a mushroom from a supplement is a different story from one growing in the yard. Wild mushrooms can include toxic species [4], so don't let her snack on anything on the lawn.
There's no official dose of reishi for dogs. What we have are study amounts and label amounts, and that's all this section covers. It's not a dosing chart.
In the beagle study, the immune changes showed up in the group getting 15 mg per kg of body weight a day, so about 136 mg for a 20 lb dog [1]. Pet Nugget lists 100 mg of reishi powder per 2 chews, next to six other mushroom powders, and the serving goes up with weight: 2 chews a day up to 30 lb, 3 chews for 31 to 60 lb, 4 for 61 to 90 lb and 5 over 90 lb. For most medium and large dogs that works out to roughly 5 to 10 mg per kg, closer to the study's lower groups than its top one. Small dogs get more per kilo, since 2 chews covers everything up to 30 lb. The study tested its own reishi powder, not Pet Nugget, so the numbers don't carry over one for one.
If you're looking at a reishi-only product, check the label for how many milligrams are in a serving, if it's a powder or an extract (and how concentrated), and which part of the mushroom it's made from.
This gets more relevant as she gets older and her list of medicines grows, which is a big part of what changes when a dog reaches middle age. Bring the label and ask:
Images are illustrative and were created with AI.
Tony Cairo·
Verified by Adam Polosetski, DVM
Tony Cairo
Miki Rapoport