Reishi Side Effects: Who Should Not Take It, and Why

Reishi is the one mushroom in coffee blends worth treating like a supplement, not a food. It is a hard, bitter bracket fungus that has always been taken as a concentrated extract, and it carries the longest list of drug cautions in the category. If you take a blood thinner, a blood pressure pill or an immunosuppressant, or have an operation booked, read the medication table before your next sachet.

What does reishi do to your body, and what does it not?

Here is the scorecard. This site makes no claims about what reishi does for you; the question is what it may do to you.

What side effects do people actually report?

Mostly a dry mouth, and mostly nothing. The one large survey asked 1,374 reishi users, all cancer patients or survivors in China, whether they had noticed any adverse effect. About one in eleven had.

Li and colleagues, 2024. The survey was partly sponsored by a reishi seller and had no control group, so read these as rough rates.

Memorial Sloan Kettering's clinician list matches it: nausea, insomnia, dry mouth, constipation, itching and vertigo have been reported. The Merck Manual adds rashes, upset stomach, diarrhea, headache, nosebleed, dizziness and loss of appetite. ^{[1] [5] [6] [7]}

Does reishi thin your blood?

It may, and the dose matters more than the headline. Memorial Sloan Kettering says reishi "can increase the risk of bleeding" and that extracts may prolong INR, PT and APTT, the three standard clotting tests. Merck names the drugs it worries about: clopidogrel, ibuprofen, naproxen, heparin and warfarin. ^{[5] [6]}

The human evidence sits at two doses, charted below: no change in clotting at 1.5 g a day, platelet aggregation inhibited at 3 g a day. One small trial and one old patient study make that a clue, not a rule.

If you take any blood thinner, including daily aspirin, mention reishi to whoever prescribes it. The conversation costs nothing.

Can it hurt your liver?

Rarely, and nearly every published case involved powdered reishi or something else going on. The NIH's LiverTox says reishi is well tolerated and has not been linked to raised liver enzymes during treatment, but has been implicated in rare single cases of acute liver injury, most of which did not adequately exclude other causes. ^[10]

Merck draws the line between forms plainly: extract for up to a year might be safe, but powdered reishi for more than a month could damage the liver. A 2025 conference abstract also reports acute liver injury in a patient taking a reishi supplement on long-term acetaminophen; only its title has been verified. Dark urine, pale stool or yellowing skin after starting reishi means stop and be seen. ^{[6] [12]}

What should you not mix it with?

Blood thinners first, then blood pressure drugs, then anything that suppresses the immune system. The full list, from the two reference sources that publish one:

Memorial Sloan Kettering also notes that reishi may trigger or exacerbate autoimmune skin disease, causation undetermined. ^[5]

Who should not take reishi without asking first?

• Anyone on a drug in the table above, especially a blood thinner, a blood pressure medicine or an immunosuppressant.
• Anyone with surgery coming up. The general supplement rule is two weeks before, per the anesthesiologists' society and an Ohio State handout that names reishi; Memorial Sloan Kettering says seven days. The one trial found no clotting change at 1.5 g a day, so this is a precaution. Tell the surgical team either way. ^{[13] [14] [15] [2]}
• Anyone with low blood pressure or a bleeding disorder, including a low platelet count. Merck advises both groups to avoid it. ^[6]
• Anyone pregnant or breastfeeding. Safety "has not been thoroughly studied", in Merck's words, and no source reports harm. An earlier version of this page said reishi is commonly advised against in pregnancy; Health Canada requires no pregnancy statement on reishi products, and Merck's reason to avoid it is missing data, not a known risk. ^{[6] [3]}
• Anyone with existing liver disease, given the case reports above.
• Anyone with a mushroom allergy. Health Canada's one required statement on reishi products is to stop use if an allergic reaction occurs. ^[3]
• Anyone with an autoimmune skin disease, per Memorial Sloan Kettering. ^[5]

Can you take reishi every day, and how much?

Daily is how it has always been taken, and Health Canada limits the dose, not the duration: 6 g of dried reishi a day as powder or ethanol extract, 15 g a day as a decoction, taken with food. Its single-ingredient monograph gives the traditional range as 1.5 to 6 g a day. The trials sit lower, at roughly 1.4 to 3 g a day over four to twelve weeks. A coffee sachet almost certainly carries a fraction of that, but we have not measured any sachet and no study has. ^{[3] [4] [16] [17]}

What does reishi feel like?

For most people, nothing. In the survey above, 90.9 percent of users reported no side effect, and the trials do not describe a felt effect. What some people notice is on the list already: a dry mouth, constipation, a worse night's sleep, an itch, a spell of dizziness. If one of those starts with the reishi, stopping is the test.

The caffeine in a mushroom coffee is a separate question, covered in the mushroom coffee side effects guide. If your blend is built on lion's mane instead, see the lion's mane side effects page.

What to do

1. Check the medication table. If you take anything on it, ask the prescriber before you start, not after.
2. Stop before surgery and tell the team. Two weeks is the common supplement rule; one week is Memorial Sloan Kettering's.
3. Prefer extract over powder if you take reishi daily; the liver cases cluster in powder taken for more than a month.
4. Take it with food, as Health Canada's directions say.
5. Stop for dark urine, yellowing skin, unusual bruising or a rash, and see a doctor.
6. Skip it entirely if you are allergic to mushrooms.

The short version

Reishi is well tolerated at the doses studied, and the side effects people report are a dry mouth and the occasional bad night. The real cautions are interactions: it may increase bleeding on blood thinners, may lower blood pressure, and is flagged for immunosuppressants and chemotherapy. Liver injury is rare and clusters in powdered reishi and people with other things going on. Pregnancy is unstudied rather than shown harmful. If you are on any of those drugs or have surgery booked, ask first.