Lecithin is not automatically unsafe for seborrheic dermatitis, but strict Malassezia screening raises a theoretical question. Lecithin is a phospholipid mixture, and laboratory work has shown that selected phosphatidylcholine lipids can support growth of Malassezia species. That does not show that cosmetic Lecithin or every finished product will worsen seb derm.

The current cosmetic safety assessment supports Lecithin within the uses reviewed, while practical Malassezia guidance describes phospholipids as debatable rather than settled. Formula composition, source and concentration can vary. If you are running a strict elimination test during a flare, an alternative may be easier to interpret; otherwise judge the finished product by your actual response.