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Certainly, you should speak with your previous spouse about his girlfriend giving your son medication that was not prescribed to him. This can be dangerous as medication is often prescribed based on weight and the girlfriend's dosage may not be the same as what should be given to your son.
You should also talk about what behaviors are occuring in their home angethd find out if there is a way to work together to solve some of those issues. You might also discuss this with your son's doctor, he should be aware that ADHD symptoms are possibly causing difficulty for your son on the weekends.
As far as the drug screening, I do not know the answer but you should be able to discuss the results with your doctor.
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