NNT values without confidence intervals may be better than nothing, but they do not tell you how likely the values, or 'point estimates', are to be true. A p value tells you that a result is seldom likely to occur by chance (less than 1 or 5% of the time). A confidence interval can tell you where the true value is most likely to be (more than 90 or 95% of the time). "You can be 95% certain that the truth is somewhere inside a 95% confidence interval". 
The NNT is the reciprocal of the absolute risk reduction.
Calculating Confidence Intervals
The pukka method is to "invert and exchange the limits of a 95% CI for the ARR" . The calculation using the confidence interval derivation for proportions is:
When not to
If the odds ratio for the result is not significant (lower odds ratio confidence interval ≤ 1), then it is unwise to bother with the confidence intervals for the NNT - see Mulrow's excellent paper  featured in Bandolier 15.
References Sackett DL, Haynes RB, Guyatt GH, Tugwell P. Clinical Epidemiology: a basic science for clinical medicine. Boston: Little, Brown, 1991.
Cook RJ, Sackett DL. The number needed to treat: a clinically useful measure of treatment effect. British Medical Journal 1995; 310: 452-4.
CD Mulrow, JA Cornell, CR Herren, A Kadri, L Farnett, C Aguilar. Hypertension in the elderly. Implications and generalizability of randomized controlled trials. Journal of the American Medical Association 1994 272:1932-8.
When even the New England Journal of Medicine has a paper with at least one number needed to treat in it, we know we are getting somewhere. This particular number is 15 - the number of men with BPH who need to be treated with finasteride for four years to prevent surgery or acute urinary retention. There is a typo in the confidence interval they quote, but excellent progress nonetheless.
Bandolier has reported on finasteride treatment for benign prostatic hyperplasia before, notably on the result of a meta-analysis showing that finasteride is effective in men with prostate volumes of more than 40 mL ( Bandolier 46 ). We also showed some NNTs and NNHs based on one RCT with two-year outcomes. We now have good data from a large RCT with four year outcomes to confirm those results .
3040 men with moderate to severe urinary symptoms and enlarged prostate glands were treated with 5 mg finasteride or placebo daily for four years in a randomised, double-blind trial of high quality. It involved clinic visits every four months for symptom scoring and clinical examination. Men with suspected or proven prostate cancer were excluded.
Finasteride did its usual job of reducing symptom scores (an average fall of 3 points from a baseline of 15), prostate volume (by 18% on average over four years, compared with a 10% increase with placebo), and increasing urinary flow (by 2 mL/sec from a baseline of 11 mL/sec). But there were also data on men who progressed to acute retention, surgery, and on adverse effects. In the Table we give both the NNTs and NNHs, and the actual rates for benefits and harms.
Numbers needed to treat and to harm with finasteride treatment for four years in men with BPH
Percentage of men with an event over 4 years with:
Finasteride reduced the number of men with acute retention, either spontaneous or precipitated by previous surgery or urinary tract infection, with an NNT of 26. To prevent a man having surgery, the NNT was 18, and for both it was 15. So 15 men with BPH have to be treated with finasteride for four years to prevent one of them having surgery or acute urinary retention. These figures are the same as, or a better than those from an earlier, smaller, trial ( Bandolier 46 ).
The report also gives good estimates of possible harms associated with treatment. For instance, in this group of men with a mean age of 64 years at baseline, about 9% became impotent over four years. But 13% became impotent with finasteride, giving a number needed to harm of 23. Other harms are given in the Table. The overall incidence of prostate cancer was 5% in each group in the intensive study.
This type of information is invaluable in informing men of their options if they have benign prostatic hyperplasia and moderate to severe symptoms. Some will want surgery, some medical treatment, while others will decide that the risks of harm from treatment outweigh the benefits for them. The great thing is that we can now begin to put numbers on the benefits and the harms - which makes it easier to give information to men, and for men to make decisions.
JD McConnell, R Bruskewitz, P Walsh et al. The effect of finasteride on the risk of acute urinary retention and the need for surgical treatment among men with benign prostatic hyperplasia. New England Journal of Medicine 1998 338: 557-63.