Which of the following drugs is NOT known to elevate prolactin levels?

Metformin is not known to elevate prolactin levels, making it the correct choice among the provided options.

Prolactin is a hormone produced by the anterior pituitary gland, and its levels can be influenced by various medications. Domperidone, a dopamine receptor antagonist, can lead to elevated prolactin levels by blocking dopamine's inhibitory effect on prolactin secretion. Methyldopa, often used to treat hypertension, is also known to stimulate prolactin release. Verapamil, a calcium channel blocker, has been associated with increased prolactin levels as well.

In contrast, metformin, which is primarily used in the management of type 2 diabetes, functions by improving insulin sensitivity and does not have a direct effect on prolactin secretion. This makes it distinct from the other options listed, which are known to impact prolactin levels adversely.