Drugs that act on the nervous system, such as the brain, and impact a person's mental state may be loosely and informally classified into categories. The following categories are not fully agreed upon globally:

Hard drugs are drugs that lead to physical and psychological addiction and potentially death. Making, selling, or using drugs other than for approved medical purposes is illegal in most countries. Hard drugs include opioids like heroin, hydrocodone, oxycodone (Oxycontin) and morphine. Benzodiazepines are another class of hard drugs and include diazepam, alprazolam, clonazepam, and lorazepam. Hard stimulant drugs include methamphetamine (meth), cocaine, and nicotine. Drugs such as alcohol and nicotine are legal to sell in many countries and are often taxed.

Soft drugs are not thought to cause physical or psychological addiction or dependence to the extent of hard drugs, however they are still considered unsafe. Examples of soft drugs are hallucinogens like cannabis, mescaline, psilocybin, LSD, ayahuasca, iboga, and DMT. While they do not cause physical addiction, some of them may still lead to psychological dependence. There is some evidence suggesting some hallucinogens have therapeutic uses. 

Some drugs do not fit perfectly as either a soft or hard drug and have characteristics of both. Examples for such drugs are MDMA (known as Ecstasy/Molly), ketamine, PCP (phencyclidine), DXM (dextromethorphan), synthetic cannabis (known as Spice or K2), and caffeine.

Effects of drug addiction on the brain 
All drugs–nicotine, cocaine, marijuana and others–affect the brain’s “reward” circuit, which is part of the limbic system. This area of the brain affects instinct and mood. Drugs target this system, which causes large amounts of dopamine—a brain chemical that helps regulate emotions and feelings of pleasure—to flood the brain. This flood of dopamine is what causes a “high.” It’s one of the main causes of drug addiction.

Although initial drug use may be voluntary, drugs can alter brain chemistry. This can actually change how the brain performs and interfere with a person’s ability to make choices. It can lead to intense cravings and compulsive drug use. Over time, this behavior can turn into a substance dependency or drug and alcohol addiction.

The effects of drugs on health 
Drugs can impact almost every organ in the human body.

 A weakened immune system, increasing the risk of illness and infection
 Heart conditions ranging from abnormal heart rates to heart attacks and collapsed veins and erectile dysfunction and blood vessel infections from injected drugs
 Nausea and abdominal pain, which can also lead to changes in appetite and weight loss
 Increased strain on the liver, which puts the person at risk of significant liver damage or liver failure
 Seizures, stroke, mental confusion and brain damage
 Lung disease
 Problems with memory, attention and decision-making, which make daily living more difficult
 Global effects of drugs on the body, such as breast development in men and increases in body temperature, which can lead to other health problems.

Legal Implications 
Charges involved for hard drugs are usually, but not always, harsher than for soft drugs. Opioid trading and trafficking are treated seriously by the law in many countries due to the potential harm. In the United States, marijuana, or cannabis, is classified as a abusable and highly addictive chemical and is heavily charged on the federal level, even if it goes against scientific evidence or laws varying by state. This is reversed for alcohol, which can cause major dependence and addiction but is normally charged with minimal or no penalty and is sold widely in the western world.

References

Other websites
Marijuana
Marsh P. et al. 2003.  Attachment, autonomy, and multifinality in adolescent internalizing and risky behavioral symptoms Dev.Psychopathol., 15: 451-467.
 Robins L.N. 1995. The natural history of substance use as a guide to setting drug policy. Am. J. Public Health. 85(1): 12–13
opioid

Drugs