system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How withdrawl from drugs causes severe illness? Sometimes death?
gpt
The worst withdrawal symptoms seem to be from drugs that are sedating: narcotics, alcohol and benzodiazepines.  

These types of drugs tend to alter the amount of signaling via various neurotransmitters and their receptors.  When one receptor is constantly activated by a drug then it will often be "down regulated" to try to bring things into balance.  Other counter regulatory pathways may also get upregulated.  But then when the drug or alcohol is suddenly stopped  - you generally get activity that is the opposite, more or less, of what the drug was causing. 

With alcohol and benzo  withdrawal this can be really dangerous - marked increases in sympathetic nervous system activity (fight or flight) occurs, heart rate and blood pressure go up, and seizures may occur.  If these reactions are severe, death can occur.

Narcotic withdrawal may cause severe discomfort - but generally not life threatening without other health problems: anxiety, sweating, aches, diarrhea, nausea, cramps and dilated pupils - many of which are from nervous system activity that is essentially the opposite of what you get with the drug itself.

In contrast, after prolonged stimulant use, stopping leads mostly to sleepy folks who are in a bad mood all the time.  But not usually very ill from a medical / vital sign perspective.