HGH stands for “human growth hormone.” The body naturally produces it, but injectable supplements exist. It helps build and rebuild muscle, which is why athletes and fitness enthusiasts have widely used it.
Mo Vaughn admitted to the Athletic that he would inject it into his knee.
Why did he use it?

Vaughn suffered a knee injury in 2001, which set him aside for the season. As he aged and recovered, the first baseman realized that he had a bad, degenerative knee and got desperate. As the power hitter neared the end of his career, he sought to salvage what remained.
For a player who built his career on strength and power, losing mobility and explosiveness was a tough pill to swallow.
The Mitchell Report

In 2007, former U.S. senator George Mitchell released a report taking a dive into the use of steroids and performance enhancing drugs in the baseball world. The report discusses the numerous players linked to PED usage.
In it, Vaughn’s use of HGH was highlighted. It mentioned three different occasions where he had purchased the supplements, all occurring in 2001 – the year he didn’t play. This fully supported the argument that he used it for recovery, rather than performance enhancement.
Affected legacy?

Will his legacy be affected? Well, in short, no.
HGH was not one of MLB’s banned substances list until 2005, two years after he retired. Additionally, the majority of his illustrious career occurred before he began injecting his knees. Vaughn had three all-star appearances, in 1995, ’96, and ’98, and not to forget his 328 career home runs.
Despite his usage of PEDs, Vaughn remains in the grey area in terms of those drugs. Regardless of this footnote in his career, his legacy still stands and his accomplishments should not be overshadowed.