Target They will then target a part of the biological process that causes the problem. This might be a missing protein, a lack of a hormone or a defective gene. In the case of asthma, scientists knew that adrenalin acted as a bronchodilator and, although not suitable itself, they reasoned that a similar substance might work.

Discovery They will then look for or design suitable compounds which might prevent, treat or cure the disease. Sometimes these compounds are based on substances already produced in the body, or are developed from plant sources. Computers are very important in medical research because they are used to help design new compounds and to select those which look most promising.

It can take several years to get a group of likely compounds, and up to 10 000 possible new chemicals might be investigated.

A potential drug will only be selected for further research if it meets certain requirements:

• it isn't poisonous
• it will be effective against the disease
• it won't break down too quickly in the body
• no major problems can be foreseen for making it on a large scale.
  

• ‘in vitro’ tests
  This stage uses techniques such as cell culture to look at biological activity and to eliminate all but a few of the original possible compounds. Compounds that have obvious toxic effects can be eliminated at this stage. In vitro means "in glass" because the tests used to be done in glass test tubes or petri dishes. Now they are usually done in plastic blocks that have 96 wells in them.

• ‘in vivo’ tests
  These are tests using laboratory animals, and they are a vital safety check. Because the bodies of other mammals work in similar ways to the human body, tests on animals show how the selected compounds are likely to react in the human body. The use of animals also gives important information about the dose level and how often to take a medicine.