People who use benzodiazepines on a regular basis may become dependent on them.

Benzodiazepines affect gamma-amino butyric acid (GABA), the major inhibitory neurotransmitter in the central nervous system. Benzodiazepines bind to receptors on the GABA–A receptor complex.

Benzodiazepines enhance the activity of GABA, effectively slowing nerve impulses throughout the body. They have sedative-hypnotic, anxiolytic, skeletal muscle relaxant, and anti-convulsant properties.

http://www.dacas.org.au/Alcohol_other_Drug_Info/Benzodiazepines.aspx

The effects of benzodiazepines can last from 2.5 hours to 160 hours depending on whether they are short, intermediate or long-acting.

As the effects of benzodiazepines begin to wear off, an individual may experience a range of effects the next day such as:

• A general hangover effect
• Reduced alertness
• Sleepiness
• Headache

• Benzodiazepines are usually swallowed.
• Some individuals also inject them; however this method carries significant risk of harms such as collapsed veins, damage to organs, stroke and even death.
• The effects of benzodiazepines may start to be felt within an hour.

It is unusual to overdose on benzodiazepines alone but if you use them with other drugs such as alcohol, heroin or methadone it is very easy to overdose and die.
Symptoms of overdose are:

• person is unable to be 'roused' or woken
• coma
• very slow breathing
• slow heartbeat
• cold clammy skin
• lips may appear bluish

Long-term, heavy use of alcohol can lead to a range of neurobiological adaptations including decreased levels of gamma-aminobutyric acid (GABA), a neurotransmitter that inhibits electrical activity in the brain, and to reduced sensitivity of GABA receptors. These effects, coupled with over stimulation of glutamate (an excitatory neurotransmitter) and an increase in glutamate receptors are thought to contribute to the Central Nervous System hyperactivity which characterises many of the symptoms of alcohol withdrawal following reduction or cessation of use.

Short-term risks include:

• accidents
• driving while intoxicated
• unwanted or unsafe sex
• injury
• interpersonal conflict
• aggression and violence

Tolerance (dependence)

People who drink heavily on a regular basis may become dependent on alcohol. They may also develop a tolerance to it, which means that they will need to drink larger amounts of alcohol to get the same effect. Tolerance develops due to increased metabolism in the liver, and changes to receptors in the brain. 

• Increases the inhibitory effects of GABA and decreases the excitatory effects of glutamate. 
• Reinforcing effects probably related to increased activity in mesolimbic dopamine pathway.