The use of blinding is traditionally associated with the use of placebos in which one group of patients receive the active treatment, and the other receives an inactive treatment or 'placebo' which must be identical in appearance and packaging and, if given orally, taste. The intention in using placebos is to eliminate the impact of the so-called placebo effect; patients may feel they have improved simply because they are being treated - or believe they are. Therefore, if an open trial compares a group allocated a new treatment with a group who receive no active care, and a benefit is seen in the treatment group, it may not be clear how much of the effect is attributable to a genuine, physical, impact of the treatment on the disease and how much is a psychological effect of simply 'being treated'. If, instead, the 'no active treatment' group receive a placebo, the difference in treatment effect between the two arms cannot be accounted for by the placebo effect, since it is affecting both arms. Of course, exactly the same problem may occur if a trial is comparing two or more active treatments. In this case, it is possible to conduct a doubleblind trial without the need for placebos, provided that the treatments can be made indistinguishable. Trials can in principle sometimes be blinded even ifthe treatments are given in very different ways - for example it is possible to compare an oral drug versus an intravenous drug using the double dummy technique. Here each patient is given two treatments, one group receives the active iv treatment and an oral placebo while the other receives the active oral agent and iv placebo treatment.

While discussion about placebos often focuses on drug therapy, in principle the need to consider blinding applies equally to any form of treatment. Ethical issues may, however, determine the extent to which treatments can be blinded, since one must avoid putting a patient at risk through any procedure which is unlikely to benefit them in any objective sense.

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