Clinical trials have several phases - Phase 1 involves testing the drug on healthy patients to check whether it is safe for humans (only later is the drug trialed on sick people). If you are healthy, you can make some money participating in these trials.
Healthy men between ages 18 to 45 are most in demand, and they usually pay £100 to £300 a day, some residential and some non-residential. However there are trials for people of all ages.
Note that sometimes trials go wrong, but the firms concerned should compensate you should this happen. Before you sign up, ask them what happens when things go wrong and what their procedure for compensation is. Then give this information to a trusted family member with instructions for what to do. The incidence of something going wrong is rare, but it does happen. The British Medical Association provides information and guidance about the regulations surrounding clinical trials.
Here is a list of current trials: