RLO: NNT/NNH: Numbers Needed To Harm

School of Nursing and Academic Division of Midwifery

RLO: Numbers needed to treat (NNT) and numbers needed to harm (NNH)

• Intro
• NNT
• Interpretation
• Comparing Treatments
• NNH
• Feedback
• Resources

Numbers needed to harm

This way of presenting information about drugs can also be used to express the measure of potential harmful effects of an intervention. In this case the control event rate and the experimental event rate are calculated as the percentage of individuals in respective treatment groups that show a harmful effect of the intervention.

In this case subtracting the control event rate from the experimental event rate gives us a measure known as absolute risk INCREASE (ARI). From this the value known as the number needed to harm (NNH) can be calculated by dividing 1 by the absolute risk increase, and again multiplying by 100 when the ARI is expressed as a percentage. NNH shows how many individuals would need to be treated with the drug in order for 1 to show the harmful effect.

Suppose both Headeeze and Noache can also cause harm and increase the number of major bleeds in patients. The control and experimental event rates for the incidence of bleeds with Noache and Headeeze is shown in this table.

The ARI and NNH for Headeeze have already been calculated. The NNH for Headeeze is 13 (i.e. 13 people need to be treated with Headeeze for 1 person to experience a bleed / harm. Calculate the absolute risk increase and the numbers needed to harm for Noache.

Does this change your view of which drug to use? (Click on the drug you would choose).

It is important to balance the benefits of an intervention with the potential harms and incorporate both in the decision making process.

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RLO Transcript (Rich Text Format 45kb)

© 2007 School of Nursing and Academic Division of Midwifery, University of Nottingham

Developer: Liz Hilton

Content author: Briony Leighton

RLO released:30 January, 2007

Page last updated: 18 October, 2013