This article is from the Impotence FAQ, by Sine Nomine llama@drizzle.com with numerous contributions by others.
Your urologist should show you, but a few important points to note:
* Injecting further toward the tip of the penis (generally not more
than 3/4 the length away from the body, though) can help make sure
the medication stays in the cavernosa longer.
* It can also help to use your hand or a cock ring to get a tourniquet
effect at the base of the penis. Leave the tourniquet in place for a
few minutes.
* After injecting, gently massage the penis to distribute the
medication evenly.
* Try to have the needle at a 90-degree angle to the penis when
injecting. If you miss the cavernosum, not much is going to happen.
 
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