Dietary nutrients or lack of them can cause dark knuckles. Research shows that knuckles pigmentation might be an external sign of a vitamin B-12 deficiency. Other symptoms that might showcase include weakness, constipation, etc.
Research shows that another cause of dark knuckles might be linked to specific drug use as they are highly likely to cause hyperpigmentation. They include anticoagulant, specific types of antibiotics, non steroidal anti-inflammatory drugs (NSAIDS), and oral contraceptive pills.
This condition causes darkening and thickening of patches of skin over different parts of the body such as knuckles, joints etc This condition is harmless by itself and not infectious although some people might develop itchiness in the affected parts.
Acanthosis Nigricans can indicate presence of deeper health issues such as diabetes, hormonal imbalances or even cancer, so if you notice your knuckles or patches of your skin getting darker suddenly and itchiness persists, see your nearest health consultant.
Most lightening products consist harsh substances like lipohydroxy acid which is derived from salicylic acid. The acid whitens the skin fast by breaking down melanocytes which produce melanin, the pigment that gives the skin its natural brown color, which the skin reacts to and in really pigmented areas such the elbows or knees. It fights it by producing more melanin in those areas.