The German cockroach, Blatella germanica, is a pest of human habitats distributed throughout the world. Boric acid (BA; H₃BO₃), in the form of powder, pellets, or blocks, has been used to control German cockroaches. Gel baits are also considered highly effective for controlling German cockroach populations. However, BA is not currently available as a gel bait. Thus, the present study was carried out to determine the best concentration of BA to formulate as a gel bait, and measure its direct effectiveness and the horizontal transfer of the active ingredient (AI) through cannibalism in adult German cockroaches (domino effect). Effectiveness of laboratory-prepared BA gel baits (lab baits) was analysed, and the efficacy and horizontal transfer of the most effective lab bait was compared to four commercial baits. Among three tested lab baits, the bait prepared with 40% BA (lab bait A) resulted in significantly greater cockroach mortality and a lower lethal time (LT₅₀) value than other lab baits. When compared with commercial baits, there was no significant difference between lab bait A and BA baits in the amount of bait consumed and the detection time. Similarly, mortality of cockroaches caused by lab bait A was equal to mortality caused by the commercial BA bait. Percentage of bait unfed adult cockroaches killed by transferred BA from lab bait A fed and dead adult cockroaches (secondary mortality) was significantly greater than that caused by both commercial BA bait and fipronil bait.