This is an advert from the National Youth Anti-Drug Media Campaign ("above the influence"). This is one of a series of posters, that depict teenagers who take drugs as rats and slugs, who are sharing rat poison and snorting salt, respectively. It aims to present a somewhat funny and memorable message that only vermin take drugs -and so, you should not be one of them.
Firstly, the makers of this ad seem to be shooting themselves in the foot by linking the target of the persuasive message to 'vermin'. This as may be effective in deterring teenagers who do not use drugs from starting drug use who do not desire to be vermin, but this may be deemed offensive to current users and thus would fail to appeal to them.
The tagline of the advert uses a rhetorical question, asking "What's the worst that could happen?". The use of rhetorical questions in advertising aims to promote a deeper processing of message content, which results in an increase in persuasion when the message is strong, but a decrease in persuasion when the message is weak (Burnkrant & Howard, 1984). Research shows that many adolescents underestimate the likelihood of bad outcomes occurring, and according to Elkind's concept of adolescent egocentrism, adolescents see themselves invulnerable to threats of death (death is a thing that "will happen to others but not to him" (p.1031)). Thus, adolescents likely will reject the suggestion of the ad that they might accidently kill themselves by taking drugs, making the message content of the metaphor weak, leading to decreased persuasion. In response to "What's the worst that could happen?", many would likely respond, "actually, not a lot". Indeed, a 5 year study by the Government Accountability Office highlights that this as campaign failed in reducing youth drug use, and resulted in weaker anti-drug norms and increases in adolescent perceptions of the prevalence of illicit drug use (http://www.gao.gov/new.items/d06818.pdf).
Burnkrant, R. E., & Howard, D. J. (1984). Effects of the use of introductory rhetorical questions versus statements on information processing, Journal of Personality and Social Psychology, 47, 1218-1230.
Elkind, D. (1967). Egocentrism in adolescence. Child Development, 38, 1025-1034.