by increasing the levels of available Serotonin (a neuro-transmitter). Depression is associated with low levels of Serotonin, and that's why Fluoxetine helps. The reason your doctors have asked you to gradually step up the dose is because by doing so, you can avoid any immediate side effects of the drug, the drug can achieve steady state concentration in your blood gradually. In other words, the intention is for your body systems to get used to the drug gradually.
Hope this helps.
Abhijeet Deshmukh, MD