A side effect is an effect that is secondary to the one intended.

Side effect or side effects may also refer to:

• Side effect (computer science), a state change caused by a function or expression
• Side effect (medicine), an unintended effect of the use of a drug

• Side Effects (Allen book), a 1986 collection of short stories by Woody Allen
• Side Effects (TV series), a 1994 Canadian TV series
• "Side Effects" (Ben 10 episode), a 2006 episode of television series Ben 10
• Side Effects (Bass book), a 2008 nonfiction book by Alison Bass

• Side Effects (2005 film), starring Katherine Heigl
• Side Effects (2013 film), directed by Steven Soderbergh
• The Side Effect, a 2014 film directed by Ti West
• Side Effects, an Indian romcom film series, including
  • Pyaar Ke Side Effects (2006)
  • Shaadi Ke Side Effects (2014)

• Side Effect (band), a 1970s soul band
  • Side Effect (album), a 1975 album by R&B group Side Effect
• The Side Effects (band), a 1980s indie rock band

• Side Effects (Dallas Smith album), 2016
  • "Side Effects" (Dallas Smith song)
• The Side Effects (album), a 2019 album by Coldrain

• "Side Effects" (Mariah Carey song), 2008
• "Side Effects" (The Chainsmokers song), 2018
• "Side Effects", a 2019 song by Stray Kids
• "Side Effects", a 2023 song by Becky Hill and Lewis Thompson

• Side product
• Side reaction
• By-product
• Causality
• Unintended consequences