A multivitamin injection is generally not recommended as a first step for your symptoms. Weakness and headaches lasting three days are more commonly caused by dehydration, viral infections, stress, lack of sleep, or blood pressure changes, rather than an acute vitamin deficiency. Multivitamin injections are typically reserved for patients with severe, clinically diagnosed nutrient deficiencies or malabsorption issues. I advise you to focus on resting, drinking plenty of fluids, and eating balanced meals. Please consult a healthcare professional for a proper physical evaluation to identify the exact cause of your symptoms and receive safe, targeted treatment.