Physical dependency and tolerance do not necessarily mean addiction. You may feel withdrawal symptoms if you abruptly stop using a drug, after becoming physically dependent. When a drug's effectiveness is reduced over time, it can cause tolerance.
                
 
What kind of changes occurs in the brainse of someone who has taken drugs?
Majority of drugs act on the brain's "reward system," which produces pleasure and releases dopamine. A healthy reward system can motivate people to take the necessary actions for survival. Dopamine spikes within the reward circuit are responsible to reinforcing dangerous, but enjoyable, behaviors like drug use. Therefore, people engage in the same behaviour again.
 
Not only are heroin and cocaine addictive, but so is cocaine and other illicit drugs. Many legal drugs can lead to addiction. These include alcohol, nicotine and sleep and anxiety medication.
The addiction to opioids and other narcotic drugs can be caused by both legal prescriptions or illegally obtained via illicit channels. The United States is experiencing a pandemic of this problem. Opioids were responsible for two-thirds (or more) of all deaths from drug overdoses in 2018. 
                                            
                                             
Relapse is the relapse of drug abuse after an abstinence program. Relapse indicates the need for additional or alternative treatment. 
  
What happens to the brain if someone uses drugs?
                                             
Addiction is different from tolerance and physical dependence. If you suddenly stop using a substance, after becoming physically dependent on it, withdrawal symptoms could occur. A state of tolerance is when a drug's effectiveness decreases over time following a prescribed dose.
If you use opioids for pain relief, you could become dependent on them or develop a tolerance. This does not mean that you are dependent. Only a small number of people will become addicted to opioids even if they are prescribed correctly by a doctor.
Your brain is programmed so that you are motivated to seek out situations that feel similar to the ones that make you happy. These acts will motivate you to do them again and again. 
Why does one person develop a drug dependence while another doesn't? A person's risk of developing a drug dependence is not determined by one factor alone. Addiction risk is affected by many factors. A person's risk factors are more likely to lead to addiction.
Biology. The inherited DNA of half the population determines a person's vulnerability to addiction. Addiction risk and drug abuse can be affected by gender, race, or other mental conditions.
Environment. The environment is not just about family and friends. It also includes the economic status of the individual and their overall quality life. Peer pressure and early exposure to drugs, stress, parental supervision, and other factors can all have an impact on the likelihood of someone becoming addicted.