An injection puts the drug straight into your body, so it gets into the blood stream very quickly. Hard drugs like heroin are often injected.
The animation shows the blood circulating around a simplified circulatory system. When an injection is given the drug is seen circulating round in the blood.
Injections make sure that a drug is carried all around your body very fast. They can be very dangerous because the body gets a high dose of the drug very suddenly which can cause your heart to stop. Once the drug gets into your blood stream it is carried to every part of you within a very short time indeed. It will affect all of your body. It reaches your brain and all your other body organs. This is one of the reasons why drugs are so dangerous.