Even an adult child who is having problems with gambling, alcohol or drugs should probably not receive large inheritances outright, and should certainly not be named as your Power of Attorney (the person who would be managing your property in the event of your disability.) If an adult child is named, and there has been a severe change in their circumstances, you should review your estate plan.
While it can be distressing to admit, you must let your attorney know about these issues when you are planning your Wills, naming beneficiaries on insurance policies or retirement accounts, and when choosing agents to serve under the various documents. Your attorney can let you know about the various strategies that exist to make sure that your child is protected in the event of your death - both from the danger of having access to no money, and the danger of having too much.
(For information on the support systems available in Massachusetts for parents of addicts, visit http://learn2cope.org/ .)