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When it becomes difficult for someone is addicted to gambling to quit. I think its not just about quitting gambling he has to do, if he as any other thing he is addicted to he should first quit on those ones because quitting those ones might make him lose interest automatically in gambling.
Being addicted to alcohol does not mean that he is also addicted to gambling because there are some people who drink alcohol but are not interested in gambling and vice versa, so if there are people who are addicted to alcohol and then gamble, it does not mean that the person will stop gambling when they stop drinking alcohol, because in my opinion, gambling addiction is not caused by drinking alcohol or smoking marijuana, but most people gamble purely from ourselves who want to get pleasure from gambling.
There are many types of addiction, gambling is one of them, but i agreed with you, if someone addicted in smoking there was no connection with gambling, on the other hand gambling addiction isn’t connected with alcohol addiction, so if anyone stop one habit that means other one will be stop automatically, that's not like that but gambling addiction is most dangerous, that can destroy a man or a family by financially.