You should ask your doctor these questions, but here's a layman's opinion: your body may develop a tolerance to any drug over time, reducing it's efficacy.  The frequency & dosage of using the drug are both factors.  Fortunately there are several ED drugs available, as well as shots.  

  Buy a pill-cutter for your viagra, & take the minimum dose that gives the desired effect.  Your doctor may change your meds over time, if the viagra is no longer effective.  I'm skeptical about the risk of mental/physical dependence on viagra, it's not an addictive drug.  Perhaps you mean psychological dependence, but that has more to do with you, not the pill.  

  My experience with the blue pill, based on my doc's recommendation: take half a dose an hour before sex; take the remaining half dose right before sex.  It works best for me on an empty stomach, chased with black coffee.  Good luck.  --z