Number needed to harm (NNH)

NNH represents the number of people that need to be treated for one adverse event to ocurr.

1. Formula: NNH = 1 / ARI
• NNT: number needed to harm
• ARI: absolute risk increase
• ARI = AR: atributabe risk
2. Classic USMLE Question:
• • "You are asked to analyze survival data after six months of treatment with a drug X. The results are given below:
                                             Alive                Dead
    Treated +                             10                  15
    Treated -                              10                  30
    
    What is the number needed to harm for drug X?
  • Answer
    NNH = 1 / AR
             = 1 / (a / a+b) - (c / c+d)
    
    a / a + b = treatment group = 10 / (10 + 15) = 0.40
    c / c + d = placebo group = 10 / (10 + 30) = 0.25
    
    NNH = 1 / (0.40 - 0.25) = 1 / 0.15 = 66.6 or 67