Nintendo makes clear on its own website that there are aspects of the Nintendo Wii that users must be aware of. The link to their webpage is here.

Alternatively please find a summary below along with some other considerations if you are using the console for therapy or rehab purposes.
(Information taken from the weblink above).

Physiological Cautions

Epilepsy / Seizures
Some people (about 1 in 4000) may have seizures or blackouts triggered by light flashes or patterns, and this may occur while they are watching TV or playing video games, even if they have never had a seizure before.

Repetition injuries
-Wrist injuries
-Eye strain
-Aching Muscles

Playing video games can make your muscles, joints, skin or eyes hurt and could lead to problems such as tendinitis, carpal tunnel syndrome, skin irritation or eyestrain.

Motion Sickness
Playing video games can cause motion sickness in some players. If the user experiences dizziness or nausea when using the Wii console, stop playing and rest. Do not engage in other demanding activity until you feel better.

Frequency Interference
The Wii console and Wii Remote can emit radio waves that can affect the operation of nearby electronics, including cardiac pacemakers.

Other Cautions
Damage to surrounding environment & equipment
-Television
- Console
- Battery Leakage
- Avoid mixing new and old batteries Electric Shock

It is recommended to not use the console during a thunder storm.

Advice to maximise the experience of using the Nintendo Wii Sit or stand as far from the screen as possible.

Play video games on the smallest available television screen.

Do not play if you are tired or need sleep.

Play in a well-lit room.

Avoid excessive play.

Take a 10 to 15 minute break every hour, even if you don't think you need it.

IIf hands, wrists, arms or eyes become tired or sore while playing, or if the user experiences symptoms such as tingling, numbness, burning or stiffness, stop and rest for several hours before playing again.