... addiction itself is a mental health problem...
Can you explain further? ...
Addiction is defined as a psych diagnosis. It easily could be considered medical. First, let's define physical dependence versus addiction.
"Withdrawal" are the physical dependence part. Your body really reacts badly to lack of the chemical of choice. For alcohol it's seizures and/or Delirium Tremens, and can be life threatening. For opiates it's diarrhea, vomiting, stomach cramps, and is usually not life threatening assuming the person can keep hydrated. For meth and other amphetamines, it's prolonged sleep, like for two weeks - not life threatening except in rare cases of heart arrhythmia. So it is very physiological in nature. Coffee has physical dependence without true addiction.
"Addiction" is the actual anatomical and biochemical changes in the brain. The brain itself is addicted. This includes changes to numbers and distribution of dopamine receptors. All the big drugs land on dopamine receptors, as far as I know. SO in that regard, the addiction is the same. Where it differs, is the production of a protein called Delta Fos B. Delta Fos B is responsible for cravings. It is also responsible for hyper-potentiation after a period of abstinence. In other words, after abstinence, Delta Fos B makes the next dose feel better than it ever has. Hence, recidivism rate is tied to levels of Delta Fos B. You produce Delta Fos B long after cessation - like 5-10 years. You will be addicted long after physical dependence has resolved.
Nicotine is worst, followed closely by methamphetamine. Then modern lab opioids like oxycontin. It's a toss up for wimpy opioids like opiuim, and alcohol. By pharmacological standards, pot is minimally addictive, despite the fear in the public.
What will kill you? Nicotine followed by alcohol. IV drug users die due to dirty needles and sepsis, but rarely due the drugs themselves. Although opioid OD is clearly on the rise due to vastly more potent lab opioids.
The Surgeon General of the good 'ole USA states "there is no safe dose of nicotine". So put that in your pipe and smoke it!