It is not uncommon to get swelling at the injection sites, which is usually just from fluid being injected. This absorbs within 30 minutes or so. Some patients are sensitive to injections or have mild bruising and can experience swelling for a longer period of time, but that is unusual.
Occasionally patients complain of swelling or puffiness in the upper eyelids after treatment of the forehead, but this usually is just due to a heaviness of the upper eyelids since the brows aren't lifting as much as they were before treatment. True allergic reactions are possible but extremely rare.
Temporary headaches after injection are also experienced by a small number of patients. Tylenol usually helps a great deal, and symptoms generally resolve in a few days at the most.