Side-effect

Weight gain is a common side-effect of certain psychiatric medications.

Source: tatoeba (11118220)

Komarova notes that everyone who studies cancer knows that genetic instability (and the mutations it causes) are important for cancer cells: cancer couldn't spread without it. It's not so clear why this mutation rate slows down in later stages of the tumor. She says that this has been observed experimentally, but researchers can't decide whether it's important for cancer growth or if it's just a side-effect of cancer.

Source: tatoeba (12250728)

Gynecomastia is estimated to affect anywhere from 30 to 70 per cent of males at some stage of their lives. And "gyne" or "gyno", as it's commonly known, is not only an unwelcome (and ironic) side-effect of anabolic-androgenic - that is, muscle-building and masculinising - steroids: it can also be caused by other hormonal disruptors such as alcohol, cigarettes, cannabis, obesity, puberty (usually temporarily) and old age. But [Tom] Powell says his gyno was "99 percent" likely due to steroids. […] The new generation of lifters are disparaged as “SARM goblins” for their embrace of the new generation of IPEDs called Selective Androgen Receptor Modulators, which supposedly provide most of the benefits of steroids and fewer of the side-effects, and can be bought online on sites like eBay and Depop.

Source: wiktionary

A common and beautiful side-effect of high solar activity is the Aurora Borealis - the northern lights that dance across Arctic skies.

Source: wiktionary