In pigs, giving GnRH‑like drugs (whether by injection or spray) helps line up ovulation for timed insemination, but it doesn't boost pregnancy or farrowing rates and actually trims litter size a bit. The way the drug is delivered (injectable vs non‑injectable) makes no real difference.

Gonadotropin-releasing hormone (GnRH) analogues are widely applied in swine production to synchronise ovulation and so facilitate fixed-time artificial insemination (FTAI). Both injectable and non-injectable formulations are available; however, their comparative effects on reproductive performance remain unclear. This systematic review and meta-analysis, conducted in accordance with PRISMA guidelines, evaluated the impact of GnRH analogues on conception rate (CR), farrowing rate (FR), weaning-to-estrus interval (WEI), piglets born per litter (PBL), and piglets born alive (PBAL). Random-effect models were used to estimate pooled mean differences (MDs) with 95 % confidence intervals (CIs). Heterogeneity was assessed using the I² statistic, and subgroup analyses compared injectable versus non-injectable regimens. The effect of a subgroup was analysed by administration route, insemination protocol, frequency, or AI method. Across the 27 included studies, GnRH analogue administration did not improve CR (MD = -0.00, 95 % CI: -0.05 to 0.05) or FR (MD = -0.02, 95 % CI: -0.06 to 0.01). Subgroup analyses showed no significant differences between injectable and non-injectable routes for any outcome, and heterogeneity remained low (I² = 0 %). A modest reduction in total PBL (MD = -0.33, 95 % CI -0.57 to -0.08) was evident without effect of subgroup. Although PBAL were slightly reduced (MD = -0.52, 95 % CI -0.82 to -0.22), particularly in subgroups by insemination protocol and insemination frequency there is no biological explanation for this finding. These findings suggest that while GnRH analogues are effective for synchronising ovulation, their influence on litter size is biologically modest. Importantly, non-injectable formulations demonstrated comparable outcomes to injectable approaches, supporting their potential as practical, less-invasive alternatives in commercial settings. To enable robust and parity-specific recommendations, further research should comprise adequately powered, parity-stratified randomised trials in gilts and primiparous (P1) sows. The effects of conventional and post-cervical routes on single FTAI should be assessed. Administration of standardised GnRH analogue dose and timing should be carried out to verify ovulation, control sperm numbers, and pre-specify outcomes (CR, FR, WEI, PBL, and PBAL).