Which of the following is an IV medication that has a narrow therapeutic index and requires periodic laboratory monitoring to prevent patient harm?

Varenicline is a medication used to aid smoking cessation by reducing withdrawal symptoms and blocking the effects of nicotine. While it plays a critical role in managing tobacco dependence, varenicline does not possess a narrow therapeutic index necessitating routine laboratory monitoring for safety.

Venlafaxine is an antidepressant belonging to the class of serotonin-norepinephrine reuptake inhibitors (SNRIs). Unlike medications with narrow therapeutic indices, venlafaxine's dosing is generally adjusted based on clinical response rather than requiring frequent laboratory tests to prevent harm.

Valacyclovir is an antiviral medication commonly used to treat herpes virus infections, including genital herpes and shingles. While adherence to prescribed dosages is important for clinical efficacy, valacyclovir does not exhibit the narrow therapeutic range characteristic of drugs like vancomycin that demand close monitoring.