Hi Cecilia,

I would say nothing wrong using an expired medium - in any cases it is not becoming bad over night just because an expiration date in written down. However, you may indeed experience a progressive decreased transfection efficiency - up to you to see if it is critical or not for what you are doing. This being said keep in mind that some culture reagents are not suitable for cells when getting old (amino acids are especially sensitive) and this might affect cell growth or even induce toxicity. 

Hope this helps. Good luck!

Since DMEM is applicable , what is the advantage of using Opti-MEM in DNA and RNA transection?

@Yuan Li the advantage of the OptiMEM is that it has no serum in it while the DMEM is supplemented with 10% usually.