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Abstract In big bang nucleosynthesis (BBN), the light matter abundance is dictated by the neutron-to-proton (n/p) ratio which is controlled by the standard weak processes in the early universe. Here, we study the effect of an extra particle species ( $$\chi $$ χ ) which co-annihilates with neutron (proton), thereby potentially changing the (n/p) ratio in addition to the former processes. We find a novel interplay between the co-annihilation and the weak interaction in deciding the (n/p) ratio and the yield of $$\chi $$ χ . Large co-annihilation strength ( $$G_D$$ G D ) in comparison to the weak coupling ( $$G_F$$ G F ), potentially can alter the number of nucleons in the thermal bath modifying the (n/p) ratio from its standard evolution. We find that the standard BBN prediction is restored for $$G_D/G_F \lesssim 10^{-2}$$ G D / G F ≲ 10 - 2 , while the mass of $$\chi $$ χ being much smaller than the neutron mass. When the mass of $$\chi $$ χ is comparable to the neutron mass, we can allow large $$G_D/G_F ~(\gtrsim 10^2)$$ G D / G F ( ≳ 10 2 ) values, as the thermal abundance of $$\chi $$ χ becomes Boltzmann-suppressed. Therefore, the (n/p) ratio is restored to its standard value via dominant weak processes in later epochs. We also discuss the stability of the new particle in an effective theory framework for co-annihilation. Further, the co-annihilation interaction generates elastic scattering of $$\chi $$ χ and nucleons at the next-to-leading order. This provides a way to probe the scenario in direct detection experiments, if $$\chi $$ χ is accidentally stable over cosmological timescale.