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Addiction not something that can be easily controlled most time it happens without one noticing it , maybe until you have gotten to a point of it becoming obvious, for instance can't start without gambling and stuff , one thing I keep telling people apart from use what you can afford to lose , is not getting addicted to gambling, because getting addicted with what can ruins you so bad.
I believe that is over-simplified, though. It is also very cliché for anyone to tell others not to get addicted to gambling (as if anyone actually wanted to get so bad and lose money because addiction). It is not enough for most people to get told to be careful, they will continue to make the same mistakes over and over again, there is when education is supposed to be used against ignorance and the false hope of making money off something which is a business model, not a charity.
If you want people to pay attention to your warning, you just don't tell them to be careful and walk away, one is also supposed to send them cautionary tales of people who fell in disgrace because their dependence to gambling and how those people used to be before and after they started to lose control. It may be not the most orthodox way to approach novice gamblers, but it is certai my better than giving them a canned and generic warning with no substance.