I have been taking 20mg prozac for about 8 weeks. it makes me feel very tired. will this get better or should i ask my doctor to change my prescription?

I assume you use prozac for depression, you did not say. You could ask for a change in prescription, however you may want to ask for psychotherapy in addition to medication. Taking care of disease goes beyond just taking medication, sometimes you may want to remove the weed by the root. You should look at you life circumstances that could be modified to help you enjoy your life.

Urge you to communicate your challenges with your doctor. Numerous anti-depressant medications are available and many likely would be better tolerated choices with equal or better success. Perhaps seek a second opinion.