The risk benefit balance is always a delicate one in medicine simply put, are the risks associated with a drug or treatment outweighed by its potential to prevent illness or ease pain?

Such is the dilemma facing a group of parents whose children attend a nursery in South London where there has been an outbreak of TB.

Health officials want to put the children on a six month regime of the anti TB drug Rifater (isoniazid). Unfortunately, in medicine, it's usually the case that the crueller the disease, the crueller the drug, and that is certainly the case with isoniazid.

In the Physicians' Desk Reference, the US drugs bible, isoniazid earns a special warning because it can lead to severe and sometimes fatal hepatitis. On the face of it, there is no need to worry overmuch as cases thus far recorded indicate that such a severe reaction is age related.

No cases have been reported in the 0-20 age range whereas eight cases per 1000 have been recorded among the over 65s who have taken the drug.

However, delving on around 3000 words on, in fact we discover that "safety and effectiveness in children or adolescents under the age of 15 have not been established". This means as is usually the case that the drug has never been tested on children in a controlled clinical trial.

As a result, we don't actually know if it works on children or what might happen to them if they take it. Seeing this, many a GP may have only rarely if at all prescribed isoniazid to a small child.

This may explain why zero cases of fatal hepatitis have been reported among the under 20s. So, the issue of danger may not, after all, be age related, but simply dose related. Zero cases have been reported because zero numbers have been taking it.

The problem is that health officials can hide behind spurious statistics such as these to support a decision and convince a worried parent. Of course, his heart may well be in the right place in this case, he's trying to stop an outbreak of TB but is that justification enough?

And has the risk benefit been proven?