Low blood pressure, also known as hypotension, is very common during major surgery under general anesthesia. Prolonged or severe hypotension can lead to complications such as kidney injury after surgery that slow down patient recovery. Anesthesiologists commonly administer medications called vasopressors to treat low blood pressure during surgery. These medications help raise the blood pressure back up to a safe range. Two vasopressor medications are commonly used for this purpose: norepinephrine and phenylephrine. Each of these medications has slightly different effects on the heart and blood vessels (cardiovascular system). It remains unknown which of these standard medications is better for treating low blood pressure during surgery. The goal of this clinical trial is to determine which of these two medications is better at preventing injury to the kidneys after major noncardiac surgery as well as other complications such as heart problems. Major surgeries are defined as those lasting at least two hours under general anesthesia. This trial will randomize about ten centers in North America to use either norepinephrine or phenylephrine as the primary medication to treat low blood pressure in adults undergoing major noncardiac surgery. Each hospital will prioritize one of the drugs each month, and the assigned drug will rotate each month at each hospital. No further participant involvement will be required as de-identified data are collected as part of standard medical care.