  Magnetic fields and stellar spots can alter the equivalent widths of
absorption lines in stellar spectra, varying during the activity cycle. This
also influences the information that we derive through spectroscopic analysis.
In this study we analyse high-resolution spectra of 211 Sun-like stars observed
at different phases of their activity cycles, in order to investigate how
stellar activity affects the spectroscopic determination of stellar parameters
and chemical abundances. We observe that equivalent widths of lines can
increase as a function of the activity index log R$^\prime_{\rm HK}$ during the
stellar cycle, which also produces an artificial growth of the stellar
microturbulence and a decrease in effective temperature and metallicity. This
effect is visible for stars with activity indexes log R$^\prime_{\rm
HK}$$\geq$$-$5.0 (i.e., younger than 4-5 Gyr) and it is more significant at
higher activity levels. These results have fundamental implications on several
topics in astrophysics that are discussed in the paper, including stellar
nucleosynthesis, chemical tagging, the study of Galactic chemical evolution,
chemically anomalous stars, the structure of the Milky Way disk, stellar
formation rates, photoevaporation of circumstellar disks, and planet hunting.
