Don’t put a needle in inflamed tissue
This may sound obvious, but it applies all over the body. Needles hurt anyway, so sticking a needle into an infected or inflamed area is going to hurt a lot more, and risk spreading the infection. This applies to local anaesthetic, injection of botox into an overactive bladder, or injecting steroids into a painful joint.
The example of local anaesthetic:
Lidocaine doesn’t work well in an acidic inflamed environment, and the blood vessels in inflamed tissue are dilated to improve supply and drainage to the area, meaning the systemic spread is massively increase. Finally, the effective half life plummets as it all gets washed away.