I understand your point about the cognitive issues. Dr. Kogelnik likes Famvir because it crosses the blood-brain barrier more easily than Valcyte. However, it has much weaker activity against HHV-6, CMV, EBV than Valcyte so he usually uses higher doses. Famvir has high activity against VSV (shingles virus) and herpes simplex and works at pretty low doses for them. Insurance companies will often pay for higher doses when the doctor takes the time to write them a detailed letter explaining why the higher dose is medically necessary. There is no guarantee as to whether an insurance company will approve a certain treatment, but you can't just assume they will deny everything. Of course, I am a US patient so I can't give any advice on whether socialized medical systems like UK NHS will pay for things.