People would begin to understand the personal responsibility aspect of this 'harmless' habit. By taking out the pleasure aspect possessing small amounts becomes pointless. Dealers would realise that having large quantities would mean they have to consume large quantities. Human rights? – no makes anyone deal, buy or use drugs. It is a personal choice (weakness?) with consequences. Also – if convicted prisoners would have to go cold turkey.
When people are caught in possession of drugs they should be put in a brightly lit room and have to stay there until they have consumed however much they had in their possession. They are released once they are 'sober'. Someone in possession of 20Kg of cocaine for example could therefore be in custody for months or year – the brightly lit room would remove the social pleasure of taking drugs. Small amounts for personal use would also lose the social pleasure aspect.