Nardil users how long has it been effective?

If you been on this medication, how long has its efficacy really lasted? I'm not talking about the initial feeling but in alleviating the crippling anxiety or feeling down. I hear stories of people all of a sudden say it has zero effects like taking nothing. Others say it's because they changed it in 2003 which doesn't make sense since you'd really need to go far back. Only those who been on it that long can really speak.