A 34-year-old woman, 10 weeks pregnant, presents with vomiting multiple times a day for the past 2 weeks. She is struggling to keep any food or fluids down and has lost 3 kg of weight. Clinical examination reveals dry mucous membranes and tachycardia. What is the most appropriate initial pharmacological treatment for hyperemesis gravidarum if non-pharmacological measures have failed, according to CKS?