Some time ago I found a San Francisco Chronicle story summarizing Mark McGwire's 1989 season. Since McGwire's now come out and said he started using steroids after the '89 season, this is a good time to present some excerpts from the story as it relates to steroids. As the Chronicle's David Bush explained, in April, during a game in Anaheim vs. the Angels, "he felt a twinge in his back and had to leave the game. He went on the disabled list for two weeks with an injured disk." When he came back, McGwire's timing and his swing were slightly off: he still had a fairly productive season, but he barely hit .230. In talking to Bush at the end of the season, McGwire said: "This is the most difficult year I’ve ever had. . . . I have just been pulling my head off the ball. When I stay down, I'm all right, but I haven't been able to do that consistently."
If we can assume that McGwire's telling the truth in stating that "I remember trying steroids very briefly in the 1989/1990 off season," it's likely that he tried them in order to help speed his recovery after twinging his back.