Vitamin K2 and D3 are two of the most common supplement pairings on the market, usually marketed together because D3 helps the body absorb calcium and K2 helps direct that calcium into bones instead of arteries.
For most healthy adults taking normal doses, side effects are mild or nonexistent. Problems mostly show up at high doses of D3, or in people taking blood thinners, where K2 can interfere with medication.