What would the side effects of being a shapeshifter be?

[JBCrowson]

Changing shape would be hard on the body. Expect joint problems as the shifter ages. Also, humans have better vision than most other animals, so there is a good chance the shifter would be colorblind - maybe monochrome, maybe green/red colorblind. Those bring up some interesting possibilities.

The original dungeon keeper game (showing my age here - it was released in 1995) allowed the player to 'possess' their minions and thus see the game world differently. One minion was a 3 headed hell hound - possessing it gave you a monochrome view port; the fly gave you a confusing compound view of the game world.

 

[JBCrowson]

I have a few races that have limited shape shifting.
Kat can choose to morph between human and big cat forms. I elected to have any clothes or weapons they were wearing / carrying disappear into the cat form and reappear when they revert to human form. As humans they have distinct feline traits like self grooming, sinuous movement, an aloof superior attitude, fast reactions, always landing on their feet, better night vision.
Ichthyi are merfolk who can become wholly human in form for short periods, with a max of about 18 hours without full water immersion. They tend to be naked below the waist when human, at least in their own settlements.
My centaurs can go fully human shaped (I added that as I wanted a love interest between a human and centaur to get physical and ... well as you can imagine the implied details were getting a bit icky). They usually carry a small backpack of clothes so when they transform to human they can get dressed.

Characters will comment internally on sensory and other physical differences when they morph. They remain very much the same person regardless of which outward appearance they are in.

 

[Azul-din]

What I've always done is to assume that the 'other' shape is in some way integral with the consciousness of the shifter; in other words, the human consciousness is present in the new form but is influenced in perception and thoughts by the form it takes.

 

[Azul-din]

Oh- I also wrote a couple of novels about werewolves- One of the things that came up is wolves age much more rapidly than humans, which when you think about it makes perfect sense of having the transformation occur only during the full moon. Otherwise the relative ages of human and wolf would get wildly out of line. I incorporated a legend about a Balkan king who decided to make an army of werewolves. The problem arose when he insisted that they change at any time he wished, accomplished by a highly polished silver moon and certain drugs. Of course when the 'age' of the wolf got too high the human was similarly affected and the king wound up with a troop of extremely ancient warriors.

 

[CupofJoe]

I like the way Terry Pratchett handles Werewolves specifically Angua.
During the transformation she retains her human abilities to reason but also gains the senses and urges of the Wolf [a hankering for the neighbours chickens and getting distracted by scents]. A down side was that clothes don't come with her, so when she changes back she is in the nuddie. She carries a lightweight silk dress on her necklace/collar for when she changes back so she has something to wear.

The lycanthropes in the Belgariad [all magicians or magician adjacent] also retain their human mind but their clothes and possession [especially a really big sword] sort of come with them. That bit is never explained. It is suggested that staying too long in an animal form may become permanent as your humanness slowly fades.