Control Groups

Scientific research has sometimes been defined by the fact that a control group is used so that the researcher can contrast the difference between those individuals who are receiving the intervention and those individuals who are not.

However there is a possible ethical dilemma in this situation. If we wish to test a new medical intervention we should find a group of people for whom this medical intervention is likely to deliver relief. If we follow the prescribed scientific method we would then divide the group into two subgroups; to one of which we would administer the new - to be tested - intervention and the other group we would administer some sort of placebo.

The ethical dimension arises in as far as it is generally thought to be improper to deny a medical intervention to anyone who could benefit from it.
If this trial was the very first on humans it might be argued that it had not yet been proven that the medical intervention worked at all and therefore we would not be depriving anyone of an intervention which would definitely ameliorate his or her condition.

But not all experimental research trials are done from such a basic starting point and once we have some evidence that the intervention will work does the issue of the control group not receiving the proper treatment becomes an ethics problem?