The county's deputy coroner Marguerite Elcock made the ruling at an inquest in Stourport-on-Severn, on Wednesday.
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Deputy coroner Marguerite Elcock heard her employer, Worcestershire County Council, was not aware of the overdose until after her death.
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Ms Elcock also praised Mrs Mann's husband Philip's "stoicism and love and support for his wife" which she said shone through throughout the proceedings.
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