A person who constantly consumes large quantities of cannabis into their body may appear apathetic, have a lack of energy, and perform poorly at their work or education.
Source: http://www.geology.yale.edu/~je84/mystuff/personal/more_personal/dap/cannabislte.html.
Financial Issues: A study taken in 1992 which estimated that the total economic cost of drug & alcohol abuse was $245.7 Billion - Out of that, $97.7 Billion was due to drug abuse. Of this, over 50% of the costs came from drug-related crimes (i.e. Break-ins, Car thefts, Muggings). If costs could somehow be reduced or if they were non-existent then the money ($97.7 Billion) could be put to use in much more worthy situations.
Source: http://www.nida.nih.gov/Infofax/costs.html.
Relationship Effects: Anyone who is a regular user of any illicit drug is bound to have a relationship argument sooner or later, whether it is with a friend, girlfriend/boyfriend or a family member. This will most probably be the starting point to a long and painful road in which they will suffer many horrifying experiences which may ruin their life. There are an increasing number of youth suicides which are seemingly caused by recent disagreements with a person close to that who is committing suicide.
Source: http://www.adf.org.au/drughit/youth.html .
Community Issues: While a regular drug user will inevitably cause damage to their own personal well being, they can also ruin the community safety net they live in. Examples of this are when they commit crimes to pay for their drug addiction, when they have a fight with someone over a drug related issue or and issue that would have never had arisen had the person not be intoxicated with illicit drugs, or peer group pressure where they will almost threaten somebody who they used to be close with into taking drugs "or else". .
Source: http://www.detya.gov.au/archive/schools/publications/1999/strategy.htm.
Drugs & Sports: Over the years, drugs in sports, especially at professional standards, have become an increasing issue in society today.