First if all I really wanted to say how great it is that your brother is in recovery. I wish him and your family all of the best. I am slightly confused about your question. Are you saying he said he had a memory while he was an addict and only now remembers?
I've not heard of addiction recovery or meth being related to false memories before, but that doesn't mean it never happens. Long-term use of meth can cause permanent psychological damage and brain damage and is associated with memory loss, so it does impact memory in some ways. Sometimes when we don't remember all of an event we make up things that didn't happen in order to fill in the blanks without realizing we're doing it.
Also, psychological research has found that it's actually pretty easy to implant false memories or alter real memories, even just by accident. I suppose it's possible that whatever rehab treatment your brother underwent accidentally had this effect.
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