The intense physical pain is due to addiction of the body to that medication.
The intense physical pain is likely occurring because of Ron's body is addicted to the anxiety drug and his body is not able to survive without that drug. Ron's withdrawing from the medication suddenly is not a right decision because his body is addicted and work properly in the presence of that medication.
So when he stop using the drug he experiences intense physical pain so we can conclude that the sudden withdrawing from the medication is responsible for this bad condition.
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