I've had a Prius for almost a year and was getting around 43 mpg, but.....

It could be any of many things. Or several of them combined.

If you can answer as much of this questionnaire as practical (i.e. don't delay responding just because you can't answer some right now), it will give us a lot more hints to go on:
Fuel economy complaints/queries? Please copy, paste & answer these questions, esp. if you're new

 

Our 2011 ALWAYS gets over 50 mpg in warm weather but drops to about 46 in the cold. Then we put on studded snow tires (rims and tires weigh about the same) and then the mpg drops to 42 from worse rolling resistance. Make sure your tire pressure has not dropped too.

 

Soooo...... it would be helpful to know where you live to kind of determine what we are dealing with here. (hint, hint- update your profile!)

 

it's both weather, and winter formulated gas. pump up your tyres and slow down!