Int. J. Environ. Res. Public Health 2012, 9 3. Results 195 Table 1 contains details of and references to 12 systematic reviews for tobacco cessation studies, three for alcohol, seven for cocaine, and five for opioids. Another meta-analysis of psychosocial interventions reported results for polysubstance use, cocaine, opioids, and cannabis [12]. In total, the 28 reviews reported on almost 139,000 participants. 3.1. Nicotine Information was available from 12 systematic reviews [13-24], 11 of which were Cochrane reviews, covering drug interventions and interventions including behavioural counselling, group therapy, and exercise programmes, as well as nurse or doctor interventions. Data were available on over 127,000 participants with cessation rates at six months or longer, with the bulk from reviews of nicotine replacement therapy (NRT), physical intervention, self-help, nursing interventions and bupropion. Most reviews used only properly randomised trials (one used controlled trials), used objective means to assess abstinence, and used either placebo or, in the case of behavioural interventions, used an appropriate control like a minimal intervention or brief advice. Table 2 shows the main results in terms of percentage of abstinent participants with intervention and placebo or control. There was a consistent response for placebo, of between 3% and 14%, with an overall average cessation rate of 8.4% in 57,867 participants on placebo. There appeared to be a lower cessation rate of 6.7% in 30,837 participants with placebo or minimal interventions in behavioural interventions, compared with 10.3% in 27,640 participants for placebo in drug interventions. This may reflect differences in populations studied or differences in methods, like blinding differences. Most interventions were effective to some extent, with NNTs varying between 7 (95% CI 6 to 10) for varenicline to 65 (45 to 110) for self-help. The very large numbers of participants in nicotine replacement therapy (NRT) studies [22] allowed investigation of a number of variables that might affect the results obtained with placebo. The first was size. Figure 1 demonstrates a consistent cessation rate with placebo below 20%, and predominantly below 10% with group sizes of 300 participants and above. It also shows very variable cessation rates of a few percent to almost 50% where group size was 100 or below. Figure 2 shows the cessation rates with placebo or control in nicotine, alcohol, cocaine and opiates. Table 3 examines other possible influences. Neither type of NRT product (gum, patch, inhaler, lozenge, or spray) nor duration of follow up beyond six months made any difference to placebo cessation rates. Participants from smoking clinics, and those with a high level of support in a group setting achieved somewhat higher rates of cessation, though below 20%. Twelve month studies were less effective (NNT 21; 95% CI 18 to 25) than sixth month studies (NNT 9.4; 7.9 to 12). This very large body of data, from many different interventions conducted in a variety of settings, shows that with placebo or no treatment after six months or longer the overall quit rate for smoking cessation is low, at about 10%. The information allows the conclusion that small studies can give highly variable and misleading results, especially where the group size is below 200 participants.