PQQ

PQQ is a redox cofactor found in small amounts in food, and what supplements actually contain is the disodium salt rather than free PQQ. The approved and the studied doses line up unusually well here: EFSA cleared it as a novel food at a maximum of 20mg a day, and the 12-week cognition trial that gets cited for it used 21.5mg a day in 58 people who completed. That trial reported broad improvements and no adverse events, which is one study, not a literature. EFSA's clearance covers healthy adults and explicitly leaves out pregnancy, breastfeeding and children, and that is an absence of data rather than a finding of harm. The mitochondrial biogenesis story that sells it is cell and animal work, so treat the retail claim and the human evidence as two separate things.

Stack Almanac totals your PQQ against the 20mg a day EFSA cleared, which is easy to pass without noticing when a CoQ10 combination product and a standalone capsule are both in the stack.