Agree with Old Guy, bldoink, and Txagq8 re auto injectors. I want to control the needle path.
Your goal is to inject into a corpus cavernosum, but this commonly-used diagram shows ERECT corpora; the corpus you inject is much thinner. Picture a clock face - if you're trying to hit the right corpus on the diagram, aim from a 1, 2, or 3 o'clock angle. If you're trying for the left corpus, aim from 9, 10, or 11 o'clock. There's an important nerve on the top, 12 o'clock, as well as a vein and artery - don't get near there!
The skin surface, and the second layer down have a fine network of nerves, and being wimpy about pain I try to put the needle between nerves fibers. I walk the needle around on the skin testing for a place between nerves, then push in to the second skin layer; if I get pain I start over. Eventually I'll get the needle through the skin and I'll get an indentation because it's against the tunica or tough covering of the corpus. Think sausage casing. I continue to slowly push and eventually it painlessly breaks through into the corpus. Now I test for absence of pain by pushing the plunger the tiniest distance to get the smallest possible drop of Trimix out of the needle. No pain - I'm in the corpus and the plunger should then have no more resistance to travel than if the needle were in air. If these two conditions don't hold, I push the needle in a little more and repeat. Another failure - I pull out and try another spot.
After slowly injecting (about ten seconds), I pull the syringe out and squeeze or pump my length 6-8 times, because the two corpora have a few fluid connections and this gets some Trimix into the other corpus.
I inject only once a week and alternate sides each time, and pick different spots along my length, in case the speculation that injecting causes scarring is true. This spreads out any tissue damage from the needle, although some claim that Trimix itself causes scarring. (After seven years of injecting, I have no apparent scarring.)
You must plan to titrate up slowly, only twice or three times a week, being sure that you're in the corpus. This slow procedure avoids priapism. It took me three weeks to find my best Trimix quantity. If you have a four-hour erection that you can't reduce with the antidote, go to an emergency room, where they will drain the corpora. I was told that at six hours the tissues run out of oxygen and gangrene begins. That happening, you must get your penis amputated!
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Age 78 in 2023. On testosterone replacement due to hypothalamus malfunction. (Attention depressed guys: low testosterone is a cause.) Healthy health nut but ED due to getting old. Like to keep enough cardiovascular ability to thrust for 30 min.