If I want to answer you in a jovial way, I would say our food, the excessive food we eat.
Do not forget the Americans are consuming five times more than us and 20% of their children are obese. In Germany are the most women and men with overweight compared to other countries in the European Union.

Now, to be serious, heroin is the most dangerous narcotic drug because it makes you addict even after a single use knowingly or unknowingly because its receptors will not disappear from our body after that. Then you need it for ever or some other substitution drug which could bind with the receptors always.

All substances which make you addict are bad to us, the only exception is your life partner.