Studies described in Table 11.1 show the efficacy of capsaicin (vs. placebo) on a range of outcomes, including pain using a visual analog scale (VAS), either a 0–100 scale or a 0–10 scale, as well as other outcomes including Western Ontario and McMaster Universities Index (WOMAC) pain, function and stiffness scales, patient and physician global evaluation, tenderness, grip strength, joint swelling, duration of morning stiffness, analgesic use, and adverse events.

VAS pain is the most commonly assessed outcome, and therefore we have further summarized the effect of capsaicin on pain intensity as assessed by visual analog scores (VAS) in Table 11.2. VAS scores have been compared using standardized mean differences in studies with available data (Deal et al. 1991; McCarthy and McCarty 1992; Altman et al. 1994; McCleane 2000; Kosuwon et al. 2010), using Review Manager (Revman) version 5.2 (The Cochrane Collaboration, Copenhagen). By convention (Cohen 1988), the standardized mean difference values of 0.2, 0.5, and 0.8 are considered small, medium, and large. Table 11.2 shows that capsaicin has a medium sized effect (SMD = 0.44) on pain intensity in patients with painful OA over 4 weeks of treatment. Results were remarkably consistent across trials with no heterogeneity, suggesting no dose response from different doses of capsaicin, and no difference in the effect of capsaicin on OA of different sites (e.g. hand vs. knee).

Most studies report capsaicin use over 4 weeks, with only, two studies continued treatment beyond 4 weeks (Altman et al. 1994; Schnitzer et al. 2012), both of which reported that continuing use was associated with beneficial effects. The study reported by Altman et al. (1994) demonstrated that the maximum difference between groups occurred at 4 weeks, and differences between groups remained similar between weeks 4 and 12. Schnitzer et al. (2012) report that differences in groups increase over time, even up to 20 weeks. These studies demonstrate that treatment with capsaicin is associated with benefits for treatment periods exceeding 4 weeks but that 4 weeks is probably sufficient for a therapeutic trial.

Two studies reported assessments of how patients reported the overall impression of treatment effectiveness (symptoms better, much better or completely gone) after 4 weeks. Capsaicin treated patients had a 40 % increased likelihood of reporting that their symptoms were at better after 4 weeks of treatment (Table 11.3). Schnitzer’s study reported that after 12 weeks, the Osteoarthritis Research Society International (OARSI) simplified response was 10 % greater in civamide-treated patients (Schnitzer et al. 2012), suggesting that response to treatment continues up until at least 12 weeks.