Background The fourth wave of the drug overdose epidemic in the United States includes increasing rates of stimulant-involved overdose. Recent studies of transitions leading to stimulant misuse have shown complex patterns that are not universally applicable because they have isolated individual populations or individual behaviors. A comprehensive analysis of transitions between behaviors and the associations with present-day problematic drug use has not been conducted. Objective This study aims to determine whether adults from the general population who use stimulants initiate use through a heterogeneous combination of behaviors and quantify the association between these typologies with present-day problematic drug use. Methods Individuals who have reported use of any stimulant in their lifetime were recruited from the 2021 Survey of Nonmedical Use of Prescription Drugs Program, a nationally representative web-based survey on drug use, to participate in a rapid follow-up survey about their past stimulant use. Individuals were asked which stimulants they used, the reasons for use, the routes of administration, and the sources of the stimulant. For each stimulant-related behavior, they were asked at what age, between 6 and 30 years, they initiated each behavior in a 6-year time window. A latent transition analysis was used to characterize heterogeneity in initiation typologies. Mutually exclusive pathways of initiation were identified manually by the researchers. The association of these pathways with present-day problematic drug use was calculated using logistic regression adjusted by the current age of the respondent. Results From a total of 1329 participants, 740 (55.7%) reported lifetime prescription stimulant use and 1077 (81%) reported lifetime illicit stimulant use. Three typologies were identified. The first typology was characterized by illicit stimulant initiation to get high, usually via oral or snorting routes and acquisition from friends or family or a dealer (illicit experimentation). The second typology was characterized by low, but approximately equal probabilities of initiating 1-2 new behaviors in a time window, but no singular set of behaviors characterized the typology (conservative initiation). The third was characterized by a high probability of initiating many diverse combinations of behaviors (nondiscriminatory experimentation). The choice of drug initiated was not a strong differentiator. Categorization of pathways showed those who were only in an illicit experimentation status (reference) had the lowest odds of having severe present-day problematic drug use. Odds were higher for a conservative initiation-only status (odds ratio [OR] 1.84, 95% CI 1.14-2.94), which is higher still for those moving from illicit experimentation to conservative initiation (OR 3.50, 95% CI 2.13-5.74), and highest for a nondiscriminatory experimentation status (OR 5.45, 95% CI 3.39-8.77). Conclusions Initiation of stimulant-related use behaviors occurred across many time windows, indicating that multiple intervention opportunities are presented. Screening should be continued throughout adulthood to address unhealthy drug use before developing into full substance use disorders.