If we spend hours on gaming may become violent, introvert, lose interest in the environment that surrounds them. 
An addict will always think of gaming, sacrificing other things that are sometimes more important than gaming, like studying, sports, socializing,.... 

Gaming also addiction can slow down the brain growth

And many other disadvantages

Disadvantage of playing video game

The disadvantages of playing video games are: Harmful to the eyes, playing for a long time will cause back pain, playing a lot will be addicted to not being able to focus on learning  

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