This is quoted from wiki cuz I'm being kinda lazy today, but from what I know myself, it's generally correct.
"In November 1968, Constanten was hired full time for the band, having only worked in the studio up to that point. Joe McIntire, an assistant road manager under Jonthan Reister, commented that "Pigpen was relegated to the congas at that point and it was really humiliating and he was really hurt, but he couldn't show it, couldn't talk about it." Ironically, the singer would achieve a new prominence throughout 1969, with versions of "Turn On Your Lovelight" (now fully ingrained as the band's show-stopping finale) regularly scaling fifteen to twenty minutes. He also developed an unlikely close friendship with Constanten based around their mutual aversion to psychedelics, and eventually served as his best man when Constanten wed. After Constanten's departure in January 1970 over musical and lifestyle differences, McKernan resumed keyboard duties.
In 1970, McKernan began experiencing congenital liver problems, likely exacerbated by his years of excessive drinking. After an August 1971 hospitalization, doctors requested that he stop touring indefinitely; pianist Keith Godchaux was subsequently hired and remained a permanent member of the band until 1979. Restless, the ailing Pigpen rejoined the band in December 1971 to supplement Godchaux on harmonica, percussion, and organ. Unfortunately, after their Europe '72 tour, McKernan's health had degenerated to the point where he could no longer continue on the road. His final concert appearance was June 17, 1972 at the Hollywood Bowl, in Los Angeles, California."