Risk factors for addiction are not yet well understood but reveal themselves by causing increased drug use, by increasing the likelihood that the individual will become addicted to the drug, or both. Other psychiatric disorders that have been hypothesized to increase risk of substance use disorders are bipolar disorder and anxiety disorders. However, this hypothesis has never been proved. Regardless of the status of the gateway hypothesis, soft drugs can produce serious difficulties of their own. After a period of experimentation, many regular substance abusers identify a drug of choice, but polysubstance abuse is becoming increasingly frequent.
Factors leading to selection of a drug of choice or to the onset of polysubstance abuse are complex.
Polysubstance abuse may be related at times to drug availability or fashions of drug use in the user's peer group. It may also develop from attempts to achieve desired drug interactions. For example, cocaine abusers may use heroin, anxiolytics, hypnotics, or alcohol to decrease the anxiety and agitation that frequently characterize cocaine binges. Substance use disorders are typically chronic, with courses often lasting many years. More than a quarter of the sample died during the follow-up period.