Some people say winter blues stem from a lack of Vitamin D*. In any case, experts (eg NHS) recommend we ALL take Vit D supplements in from around Sept to March, at least.
According to the NHS, Vit D supplements are safe, the toxic dose is MUCH higher than the amount in cheap supplements,
BUT cod liver oil should be taken carefully as it usually has Vit A, which is toxic at fairly low doses.
*even if you get out on sunny days,
the UVB needed for synthesis of D does not get through.