Rosacea: This condition is characterized by episodes of flushing and persistent redness, often affecting the central face but can be unilateral. It's common in older adults and can be exacerbated by various factors including sun exposure, stress, and certain foods.

Contact Dermatitis: An allergic or irritant reaction to a substance that comes into contact with the skin, such as a new skincare product, makeup, or an allergen, could cause localized redness.
Sunburn: Prolonged exposure to the sun without adequate protection can lead to sunburn, which might cause redness on one side of the face if the exposure was uneven.
Eczema (Atopic Dermatitis): Although more common in children, eczema can occur in adults and may present with red, itchy patches on the face.
Seborrheic Dermatitis: A condition causing scaly patches, red skin, and stubborn dandruff, often affecting oily areas of the body like the face.

Cellulitis: A bacterial skin infection that can cause redness, swelling, and warmth of the skin. It requires prompt antibiotic treatment to prevent serious complications.
Lupus: Systemic lupus erythematosus can cause a butterfly-shaped rash across the cheeks, but in some cases, the rash might be more localized or unilateral.
Dermatomyositis: An inflammatory disease characterized by skin rash and muscle weakness. The rash can be quite distinctive but might initially present with facial redness.

Erythromelalgia: A disorder characterized by burning pain, heat, and redness in the hands and feet, but in rare cases, it might affect the face.
Polymyalgia Rheumatica: Although it primarily affects the muscles, causing stiffness and pain, some patients might experience facial symptoms, including redness.
Sjögren's Syndrome: An autoimmune disorder that can cause dryness in the eyes and mouth but might also lead to skin manifestations, including redness and rashes.