A 52-year-old man with alcohol dependence is brought to the emergency department after being found confused at home. He has had several days of vomiting and is unable to stand without support; his capillary glucose is 6.2 mmol/L (reference range 3.5–7.7 mmol/L), and CT brain is not immediately available. He is haemodynamically stable and has no focal neurological deficit. Which one of the following should be given immediately?