https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3824348/
There is no robust data on effectiveness. However, reviews of the data that are available report that, over follow-up periods ranging from 1 to 35 years, recidivism rates of 2.5–7.5 percent were found after surgical castration compared to 60–84 percent in offenders left “untreated” (Heim and Hursch 1979; Weinberger et al. 2005).
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Chemical castration via administration of CPA and MPA has been found effective in reducing recidivism in sexual offenders with paraphilias in some small-scale, controlled studies (e.g., Fedoroff et al. 1992; Maletzky, Tolan, and McFarland 2006; Meyer et al. 1992). However, other studies found no significant effect (e.g., Hucker, Langevin, and Bain 1988; Maletzky 1991).2 Several studies have shown high efficacy rates for GnRH agonists in dramatically reducing testosterone levels and self-reported deviant sexual desires and behaviours, including in individuals who did not respond to CPA or MPA. However, no randomised controlled trials have yet been published (Thibaut et al. 2010).
Thus, for both traditional agents and GnRH agonists, evidence for effectiveness is not robust. Moreover, chemical castration appears to be ineffective in antisocial or psychopathic sex offenders who do not suffer from paraphilia (Berlin 2009), and certain comorbidities may preclude effective intervention in individuals with paraphilia (Saleh and Guidry 2003).