The HT itself doesn't cause ED but if you don't keep the penis healthy, ED will happen later when your libido returns. Most, but not all, men lose their libido very quickly on HT which means not only that you don't feel randy when sexual activity is available but also you don't get all those random erections or semis during the day & night. This leads to atrophy - basically, the spongy tissue of the penis dies off and can no longer expand and engorge.
There are a number of ways to maintain penile health - using any erection that does occur, trying to get an erection even if you don't feel randy, using a vacuum pump on a daily basis, etc. Proper vacuum pumps are available on the NHS but it is a bit of a postcode lottery; in some areas they are only available to men who have had the prostate removed, in other areas they are not funded at all. It is worth asking your onco, specialist nurse and / or GP for a referral to the local ED / andrology clinic (if one exists in your area) and if not, then ask your GP whether s/he can / is willing to prescribe.
Re Prostap / Zoladex - some oncos have a strong preference .. others leave it up to the GP to choose. Some GP practices keep all men on the same HT so that the practice nurse is skilled in delivering the depot (which can be a bit of an art). If given the choice, the differences are:
- Zoladex is given in the stomach every 28 days / 12 weeks and should not be done more than 1 or 2 days early or late
- Prostap is given into the stomach or a large muscle such as the buttock or thigh; it is given every month / 3 months and there can be a little bit more flexibility with 3 or 4 days early or late if necessary (for example, if you are away on holiday)
- some doctors prefer Decapeptyl - this is given in the buttock every three months and is available as a 6 month (actually 24 weeks rather than 26) dose as well as 3 month or one month.
Usually, you get a one month dose the first time and then go to the longer dose; as D says, this is in case of a severe reaction. Worth checking whether you will be getting your injection / depot at the hospital or GP practice. Side effects are very similar for all three drugs.