I want to share this because the Reishi community tends to emphasize the positive while downplaying the risks, and my experience shows that those risks are real.
I started taking Reishi at a high dose — 3000mg of a concentrated extract twice daily (6000mg total) — after reading about its 5-alpha-reductase inhibiting properties and potential prostate benefits. I am 58 and was looking for natural ways to manage early BPH symptoms.
For the first two months, everything was great. Urinary symptoms improved modestly, sleep improved, and I generally felt good. No side effects.
At my routine physical at the 3-month mark, my liver enzymes came back elevated. ALT was 78 U/L (normal is under 40) and AST was 62 U/L (normal is under 40). My doctor was concerned — these were not dramatically high but were clearly above my baseline of ALT 22 and AST 19.
She asked about any changes in medications or supplements. When I mentioned the Reishi, she pulled up the literature and showed me the case reports of Reishi hepatotoxicity, including a fatal case. She asked me to stop immediately and recheck in six weeks.
Six weeks later: ALT 28, AST 21. Back to normal.
I believe the issue was the dose. 6000mg of concentrated extract is well above what most clinical trials have used (typically 1800-3000mg). I was also using a product that may not have been optimally processed. I am not anti-Reishi — I think it is genuinely beneficial at appropriate doses — but this experience taught me that "natural" does not mean "harmless" and that more is not always better.
If you are taking Reishi daily, especially at higher doses: get your liver enzymes checked periodically. It is a simple, cheap blood test that can catch problems before they become serious.