A 24-year-old woman presents with palpitations and light-headedness. Her ECG shows a regular narrow complex tachycardia at a rate of 180 bpm. P waves are not clearly visible. Vagal manoeuvres (Valsalva) fail to terminate the rhythm. Her blood pressure is 115/75 mmHg and she is haemodynamically stable. According to Resuscitation Council UK (referenced by NICE), what is the most appropriate next pharmacological treatment?