The sensation of heaviness in the forehead is an uncommon side effect of Botox treatment. Luckily, it’s easily treatable. Botox is a protein that has been purified and treats wrinkles triggered by facial expression. Such wrinkles include horizontal forehead lines that appear across the forehead when raising an eyebrow, the vertical lines between the brows when people frown, and the crows feet that show around the eyes when squinting or smiling.