To build on this, Phase 1 trials are meant to determine safety - the treatment is given to a smallish group of healthy individuals to look for side-effects and

To build on this, Phase 1 trials are meant to determine safety - the treatment is given to a smallish group of healthy individuals to look for side-effects and what not. Phase 2 is a small scale test for effectiveness of the treatment and more side effects. Phase 3 is a large scale test of effectiveness and more side effects. After passing Phase 3, the treatment can probably be put out to market, and then 'Phase 4' (post-market) data can be gathered to ensure that nothing was missed.