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The general rule is every second day or 3 times a week max. But if one injection has absolutely no effect (no erection at all, either missed the spot or trimix has no effect), does that still count, or, can I have an extra injection?

The every two day/three days a week is only a general rule. You can inject a second time or inject several days in a row as long as you do not do it on a regular basis.