The present invention provides an improved method for preparing a compound of formula (IIIA), which is a varenicline synthesis intermediate. Furthermore, the present invention provides an improved method for preparing varenicline, or a salt or a solvate thereof. Moreover, the present invention provides a process for decolouring varenicline, or a salt or a solvate thereof. Moreover, the present invention provides a process for preparing varenicline L-tartrate, with an improved yield. Moreover, the present invention relates to the use of a compound of formula (V), or a salt or a solvate thereof, as reference marker and reference standard for evaluating the purity of varenicline, or a salt or a solvate thereof.
