A 27 year old man with epilepsy presents with complaints of persistent lethargy and occasional feelings of intoxication, although he does not consume alcohol. The patient is currently taking phenobarbital for the long-term management of tonic-clonic seizures. In addition, he was recently staed on a medication for the treatment of gastroesophageal reflux disease. Which of the following agents was he most likely prescribed?

Cimetidine is an H2-receptor antagonist indicated for the sho-term and maintenance treatment of duodenal and gastric ulceration, as well as gastroesophageal reflux disease. One of the primary disadvantages of using this agent, with respect to other H2-receptor antagonists famotidine and ranitidine, is that it is a relatively potent hepatic enzyme inhibitor. Therefore, this medication is likely to decrease the metabolism of other hepatically metabolized medications, such as phenobarbital. When the metabolism of a medication is decreased, the blood levels will increase, leading to an extension of the therapeutic effect and/or toxicity. Phenobarbital is a barbiturate indicated for the treatment of tonic-clonic seizures and status epilepticus. When the blood concentration of this medication increases, lethargy and feelings of intoxication may occur. Lansoprazole is a gastric acid proton-pump inhibitor indicated for the sho-term and maintenance treatment of duodenal and gastric ulceration, as well as gastroesophageal reflux disease. This agent does not affect hepatic enzymes. Ref: McQuaid K.R. (2012). Chapter 62. Drugs Used in the Treatment of Gastrointestinal Diseases. In B.G. Katzung, S.B. Masters, A.J. Trevor (Eds),Basic & Clinical Pharmacology, 12e.