Hi, thank you for your question. Just a bit more information required to fully assist you:
-What is your gross salary?
-How many nights, on average, does your son stay with you each week?
-Do you have any other children in your care, if so how old?
-What substances is she abusing?
Thank you for your question
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I am afraid that you have no right to ask your ex to account for the maintenance that you pay her.
You can calculate if you are paying the correct amount using the calculation set out here
(essentially 12% of your gross income at most)
However you can ask for her to account for any extra money that you give for specific issues.
If you believe that your ex is abusing drink or drugs and placing your son at risk you may wish to consider whether or not you wish to apply to have the child in your care
Please ask if you need further details