Common (>10%):

• Chromaturia (reddish-brown urine) — harmless, dose-dependent, not haematuria, expected from idebenone metabolites. Will mimic blood in urine and can be alarming if you don't know to expect it.
• Mild GI upset (nausea, loose stools) — usually if taken without food. Eliminated by taking with breakfast/lunch and fat.

Less common (1-10%):

• Headache — most often dose-related; usually fades after first 1-2 weeks.
• Mild insomnia if dosed late (after 5 PM).
• Mild diarrhea, dyspepsia.
• Skin rash — usually mild, idiosyncratic.
• Mental agitation / increased nervousness in a minority — Russian Noben labels list this; consistent with the "mild stimulant" anecdotal profile.

Rare-serious (<1%):

• Liver enzyme elevations / hepatotoxicity. Documented in the EMA Raxone product information at 900 mg/day clinical doses; resulted in temporary interruption or discontinuation in some patients with pre-existing hepatic impairment. Watch ALT/AST especially in the first 8-12 weeks at doses >300 mg/day. Has not been reported as a dose-limiting issue at the 90-270 mg/day biohacker dose tier, but baseline + 8-week ALT/AST is the prudent move.
• Idiosyncratic allergic reactions — extremely rare, including anaphylaxis-class events on the Raxone label.
• Seizure threshold lowering — theoretical (any quinone in the brain at high local concentration); not clinically observed at standard doses.

Specific watch periods:

• First 8-12 weeks at any dose >300 mg/day: ALT/AST baseline + at week 8.
• First 1-2 weeks at any dose: GI tolerance check. If chromaturia is alarming, normalize expectations.
• Concurrent hepatotoxic drugs/alcohol: add an additional ALT/AST check at week 4.

Doses up to 2250 mg/day have been administered in clinical Friedreich's studies; the safety profile remained "consistent" but the upper end is associated with the hepatic-enzyme-elevation tail and is well past any nootropic-tier rationale.