How to fade a disease-caused darker coloring? And freckles

Not looking for medical advice.

I have Addison's Disease, and one unfortunate symptom that can happen is darkened areas of skin. My forehead is darker, almost like a tan, compared to the rest of my face.

My freckles are much darker than they used to be, too. Not sure if this is also a side-effect from the disease.

I wear sunscreen every day for a long time so I do not believe it has to do with the sun.

I have read conflicting things about Niacinamide.

Is that what I should try? Is there something better?