This paper examines the reinforcement of sex work and drug use. Data from 92 survey and in-depth interviews were analysed to examine associations between vulnerabilities and ‘trapping’: mutually reinforcing sex work and problematic drug use. Three-quarters of participants had used drugs and half had sold sex before 18. Half had been in care and half had been homeless. One in 5 (21%) had run away or left home before 16 and 80% had been convicted for acquisitive offences. Nevertheless, these experiences did not explain trapping. There were strong associations between being trapped and convictions (81%); and being trapped and outdoor/drift sex work (92%). After adjustment for the other vulnerabilities in the logistic regression only outdoor/drift sex work remained significant: with sex workers involved in outdoor/drift sex-work having an adjusted odds ratio of over 7 (95% c.i. 1.7–28.3) of being trapped. Moreover, since the sample over-represents the extent of problematic drug use amongst indoor sex workers the study underestimates differences in trapping potential between sex markets. We conclude that outdoor/drift sex markets may reinforce vulnerability, sex work and problematic drug use. Interventions are recommended to disentangle sex and drugs markets, and reduce the sex industry's potential for exploitation and abuse.