### Description

The number needed to harm (NNH) indicates the number of patients who would need to be treated for one patient to have an adverse event as compared to an untreated patient.

Variables:

(1) number of people in control group

(2) number of people in control group who develop an adverse event during the time interval

(3) number of people in active treatment group

(4) number of people in active treatment group who develop an adverse event during the time interval

Usually the rate of adverse events in the treatment group is greater than in the control group.

adverse event rate in control group =

= (number of people in control group with adverse event) / (number of people in control group)

adverse event rate in active treatment group =

= (number of people in active treatment group with adverse) / (number of people in active treatment group)

relative risk increase =

= ((adverse event rate in treatment group) - (adverse event rate in control group)) / (adverse event rate in control group)

absolute risk increase =

= (adverse event rate in treatment group) - (adverse event rate in control group)

number needed to harm =

= 1 / (absolute risk increase) =

= 1 / ((adverse event rate in treatment group) - (adverse event rate in control group))

Interpretation:

• The number needed to harm indicates the number of patients who need to be treated to have an adverse event during the time interval relative to the control group.

• The greater the number needed to harm, the lower the risk associated with the therapy.