This has the effect, since demand has not been similarly restricted, of raising the price. Every big seizure of drugs boosts the profits of those drug dealers whose imports haven't been seized. The more that gets seized the better the prospects for the other dealers and the greater the incentive to import more. In other words, prohibition guarantees the profits of the dealers. No wonder it has failed.
Prohibition has various other effects.
It recruits a sales force of small time users who sell drugs in order to help pay for their own consumption. It boosts crime, since those who have acquired expensive addictions will steal to get the money they need. Dealers dilute their products in order to sell them for more and this increases the health risks run by drug users. Prohibition also leads to police corruption. Dealers bribe the police to ignore their activities and occasionally give them the names of other dealers, business rivals, so that the police have arrests to their credit and the dealers have reduced competition to contend with.