Trauma or Contusion: This is often the most common cause of forehead swelling, resulting from a direct blow to the forehead. The swelling can be due to blood accumulation (hematoma) or fluid accumulation (edema) in the tissues.

Cellulitis or Skin Infection: Bacterial infections of the skin can cause swelling, redness, and warmth of the forehead. This can be due to a variety of bacteria, often introduced through a break in the skin.
Insect Bite or Sting: Reactions to insect bites or stings can cause localized swelling, which in some cases can be quite pronounced.
Allergic Contact Dermatitis: An allergic reaction to substances that come into contact with the skin, such as certain metals in jewelry or components of hair dyes, can lead to swelling, redness, and itching.

Cavernous Sinus Thrombosis: Although rare, this condition is potentially life-threatening. It involves the formation of a blood clot within the cavernous sinus, which can be a complication of facial infections, including those of the forehead. Symptoms can include swelling, but also more severe signs like fever, headache, and neurological deficits.
Meningitis or Intracranial Abscess: Infections that spread to the meninges (the protective membranes covering the brain and spinal cord) or form abscesses within the brain can present with forehead swelling among other symptoms, including severe headache, fever, and altered mental status.

Sarcoidosis: A systemic disease characterized by the formation of granulomas in various parts of the body, including the skin. Forehead swelling can occur due to granulomatous infiltration.
Wegener's Granulomatosis (Granulomatosis with Polyangiitis): A rare autoimmune disorder that can cause inflammation of the blood vessels, leading to a variety of symptoms including skin lesions and swelling.
Neurofibromatosis: A genetic disorder that can cause tumors to form on nerve tissue, potentially leading to forehead swelling if a tumor develops in this area.