Um. . . . .what are you trying to inject? You need to know that it's an injection that is supposed to go into a vein. . .or a muscle.
Look, if you're trying to inject something into the blood stream, when you think you've entered a vein, then you draw the plunger back slowly while trying to not move the needle. . .and if you draw back some blood-then you're where you need to be. Inject SLOWLY.
. . .but I don't understand your question-if someone needs an injection of some sort of medication. . . then you can't just wait "a few weeks," so what's up?