EVERY show is a dress rehearsal for the next one. This adage, so familiar with thespians, holds true for drugs, too. But the note is deadlier.

Naturally, any drug must go through a rigorous series of clinical trials before being unleashed in the market. These come in phases. The first of the 3 phases attempts to determine the maximum tolerable dose of the drug in humans, and the adverse reactions, if any, to it. Healthy, adult males, are administered the new drug under the supervision of trained clinical pharmacologists, having facilities to closely observe and monitor the subject.

In the next phase, exploratory trials are carried out on a limited number of patients (10-20) to determine its possible therapeutic use, effectiveness of the dose range, and further evaluation of safety levels. These are carried out in 3 or 4 centres by clinicians who are specialists in the concerned therapeutic areas.

The phase III trials are confirmatory in nature, identifying the efficacy and safety of the drug in a large number of patients, usually in comparison with a standard drug or a placebo.

If the drug has already been marketed in other countries, phase III data is obtained on at least 100 patients in 3 to 5 centres. If the drug is a new chemical entity (NCE) and has not been marketed in any country, phase III trials are to be carried out on at least 500 patients distributed over 10-15 centres.

In addition to this, data on adverse drug reaction (ADR) observed during the clinical use of the drug is to be collected from 1000-2000 patients. The introduction of drugs which fall into either of these categories requires the permission of the Drugs Controller India (DCI).

Drugs already approved or marketed in other countries, require phase III trials. But for drugs not approved or marketed elsewhere, trials are usually allowed to be initiated at one phase earlier to the phase of the trials in other countries. For NCEs discovered in other countries, phase I trials are not allowed to be conducted in India unless similar data from other countries is available. However, such trials may be initiated in India if the NCE is of special relevance to health problems typical of India.

Sometimes, a drug which has been approved for a certain indication and is introduced through a certain route -- oral or parenteral (administered through any point other than the gastro-intestinal tract) -- is proposed to be used for another indication, and administered through by a different route. The new routing may also be combined with a dosage regimen different from the original one. Such usages need special trials. And so do a combination of 2 or more drugs which, though approved individually, are proposed to be combined for the first time in a fixed dose combination.