FLAG A nurse is providing discharge instructions to a client who has syndrome of inappropriate antidiuretic hormone (SIADH) with a prescription for tolvaptan. Which of the following items should the nurse reinforce to the client to avoid while taking this medication?
a. Acetaminophen
b. Spinach
c. Dairy products
d. Grapefruit

The vasopressin (antidiuretic hormone, ADH) antagonist medication tolvaptan helps patients increase renal water excretion without also eliminating sodium, thereby combating hypervolemia caused by decreased urination due to SIADH without causing a hyponatremic state. In plain English, it forms more pee without wasting electrolytes, despite the brain producing too much of the hormone that tells the body not the produce pee.

However, like most medications, there are some drug-to-drug interactions and drug-to-food interactions that the nurse should inform the patient to be aware of.

Before taking over-the-counter (OTC) medications, like acetaminophen (brand name Tylenol), the patient should first notify and consult their prescribing provider about the specific medication they are taking and why. This does not mean that they must avoid Tylenol though.

What should be avoided is grapefruit and grapefruit juice with this medication. This is because tolvaptan is metabolized in the liver by an enzyme known as CYP3A4. Grapefruit can block this action from happening, which then the drug remains in the blood for longer than it should and increases the risk for drug toxicity.

Therefore, option D, grapefruit, is what the nurse should reinforce the patient to avoid.