"[These webpages] give a clear explanation on how the polymerase chain reaction works and why it is useful."
Some illegal drugs (eg cannabis) are smoked. The process by which the drug gets into your body is exactly the same as when you smoke a normal cigarette. Often the drug is mixed with normal tobacco, which means you suffer the health risks of normal smoking as well as those of taking the drug.
The animation starts with normal breathing. When you smoke a drug you breathe the smoke down into your lungs – the animation shows this happening.
Once the smoke is in your lungs the drug passes out into the blood stream just like the oxygen does. It is carried all around the body. The drug affects your brain. Most drugs are addictive. They change the chemical processes in your body so that you may become addicted to them (dependent on them). If you are addicted to a drug you cannot manage properly without it. Once you are addicted to a drug, you generally need more and more of it to keep you feeling normal. When addicts try to stop using drugs they usually feel very unwell. They often have aches and pains, sweating, shaking, headaches and cravings for their drug. These are known as withdrawal symptoms.
The animation shows how the drug is taken down into the gut and then moves through the gut wall into the blood. Most drugs are absorbed into the blood in the small intestine but some also go through the stomach wall.
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.