A forehead vein can appear due to stress, physical exertion, or heat, but it may also simply be a normal personal characteristic. It is more common in lean individuals because having less fat beneath the skin makes veins easier to see.
Although it is less common, sometimes an enlarged artery along the side of the forehead can be caused by a health problem, such as temporal arteritis. In this case, other symptoms may also be present, including fever, weight loss, headache, increased sensitivity in the area, and vision changes.
If you notice that a forehead vein or artery appears larger than usual, or if you experience other symptoms, you should see your primary care provider to find out whether there is any underlying condition that requires treatment.