There are different ways that these drugs affect the body. The user can smoke (breathe in), inject, sniff (breathe in the solid), or ingest (eat) the drugs. Once they are inside and in the bloodstream the drug changes the way chemicals are sent throughout the brain. By changing the messages sent, the brain will respond in a way specific to each drug. Some drugs and their effect will be felt in minutes and only last for a few hours. Other drugs will be felt in around an hour and then their effects can last up to several days. The amount of the drug taken can also change how strongly the user reacts to it. Too much of the drug can lead to drug tolerance. Drug tolerance is when the user has taken the drug for a long time and can no longer get the same effect from only taking a small amount of the drug. This will cause the person to take more of the drug to have the same effect. Doing this will eventually lead to an overdose. Designer drugs can be highly addictive so it can be hard to stop taking them in dangerous amounts.