The failure of prohibition is entirely predictable if you think about the underlying economics. The price of the prohibited substance is artificially inflated, creating a huge incentive for criminals to enter the market in order to satisfy demand. Attempts at policing tend to be futile, because if successful, they simply push up the price and therefore increase the supply incentive. The high price also incentivises users to engage in criminal activity in order to purchase the drug. Worse still, the supplier is outside the law, so the threat of violence will inevitably be one of the methods of enforcing contracts.
The huge resources in the hands of criminal suppliers inevitably result in the corruption of legal and financial systems. Drug suppliers need to be able to invest those resources and one way is by moving them into the “legitimate” economy (money laundering). The other is to invest them back into their business, for example by paying off officials and politicians.
The negative consequences of prohibition could be avoided if the government followed a simple rule – adults should be allowed to do what they want provided it does not directly harm anyone else, even if it is harmful to themselves, and, here is the difficult bit, even if the government does not approve. And even if this principle is not accepted, it should be clear by now, after 30 years of the “war on drugs”, that it is the prohibition of drugs, not their consumption, that causes most of the harm.