Vitamin K has one of the cleanest safety profiles in supplementation. No tolerable upper intake level (UL) has been established by the IOM or EFSA because no toxicity has been documented at intakes up to ≥45 mg/day MK-4 in Japanese osteoporosis trials.

Common side effects: functionally none at supplementation doses. The dopamine.club community data for "vitamin-k" shows reported side effects clustering on insomnia/anxiety/fatigue/brain-fog (61/59/80/58 reports out of ~1500) — these are almost certainly attribution errors from co-supplementation with D3 (often the actual culprit when high-dose D3 outruns magnesium) rather than K-driven effects.

Less common (idiosyncratic, plausibly real):

• Heavier menstrual bleeding at MK-7 ≥360 mcg in occasional women. Reverses on dose drop.
• GI upset at very high pharmacologic MK-4 doses (15-45 mg) — bloating, nausea. Not seen at supplemental MK-7 doses.
• Allergic reaction to soy-derived natto extracts in soy-sensitive users. Bacillus subtilis natto-fermented MK-7 is sometimes labelled "soy-free" but verify with the manufacturer.

Rare-serious:

• Thrombotic events — theoretical concern given K's clotting role; not documented at supplementation doses in healthy adults, including at 360-720 mcg/day in long RCTs. The clotting cascade saturates at modest K intake; additional K does not push clotting harder.
• Acute hemolytic anemia and kernicterus in newborns — historically associated with synthetic menadione (K3), which is not used in modern supplements. Avoid any "K3" product; this isn't a concern with K1, MK-4, or MK-7.
• Pediatric IV K1 anaphylaxis — hospital injectable concern only.

Watch periods:

• First week of D3+K2 start: transient palpitations or calcium shifts in users with low magnesium baseline. Add magnesium glycinate 200-400 mg if seen.
• Post-surgery washout: some surgeons request 5-7 day pause pre-op; clinically minor but follow if asked.
• Pregnancy: K1 and K2 considered safe at standard doses; menadione (K3) contraindicated.