She started freebasing

pivotal · ~YTSDB ("five years back"), recalled

• When: ~YTSDB (2004) — "five years back, before Incandenza's death" (p. 237); NB the arithmetic is loose (five years back from Nov YDAU lands late 2004, after the 1 Apr death) — as printed (minor DATING PUZZLE)
• What: She began freebasing cocaine roughly five years before YDAU, before JOI's death; it was during the cooking, her hands trembling, that she first knew "she liked this more than anyone can like anything and still live" (digest phrasing, p. 237).