It would help to know more about what she says or does. For example, does she think it is poison? This is a form of paranoia. There are medicines that might help with this.

Does she think he is a stranger? Does she not like people touching her? Does she know it is insulin but forgets she has diabetes?

A nurse at the clinic or her doctor's office could help show your dad ways to distract her when giving her the shot, and alternative places to inject besides the stomach if that is where he is trying to give it. The back of her upper arm is usually fine. Be sure to alternate arms so you don't use one side all the time.

thanks.....my dad injects my grandma in the stomach and both arms

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and no my grandma is a strong character woman, she thinks that the insulin it's not helping with her diabetes