Some Fitbit Force users have been reporting a rash on their wrist. The rash is thought to be contact dermatitis due to nickel, so if you have a nickel allergy, you may want to avoid the Fitbit Force.
How do you know if you have a nickel allergy? Have you ever noticed a rash where some metal on clothes (like a button) has caused a rash on your skin? Some people report a rash on their waist from their pants button. Other people develop rashes when wearing certain types of metal jewelry.
The Fitbit Flex (which lacks the metal component and the display) hasn't caused any contact dermatitis rashes from metal because there is no metal that touches the skin.
If you're experiencing a rash from your Fitbit Force, contact Fitbit.