 Effects of Botox may spread hours to weeks after injection causing serious symptoms. Alert your doctor right away as difficulty swallowing, speaking, breathing, eye problems, or muscle weakness can be signs of a life-threatening condition. Side effects may include allergic reactions, neck and injection site pain, fatigue, and headache. Don't receive Botox if there's a skin infection. Tell your doctor your medical history, muscle or nerve conditions, and medications, including botulinum toxins, as these may increase the risk of serious side effects.