Alcohol misuse costs the Northern Ireland economy almost £1 billion a year, research showed.
Drinking too much caused lost productivity, premature death and unemployment, according Addiction NI.
Director Thelma Abernethy said it had a significant impact on mental health.
She said: “Here in Northern Ireland, it is estimated that in the region of 3.64 million working hours are lost every year due to alcohol use alone.
“This coupled with the ever-increasing use of legal and illegal drugs in the workplace will have an even bigger impact on individuals, families and business.
“It is important that businesses understand the impact of alcohol and drug use within the workplace as well as their legal responsibilities to ensure the overall health, wellbeing and safety of their workforce.”
The £900 million cost includes the bill to the health service as well as social work, the fire and police services and courts and prisons, Addiction NI said.
A summit organised by Randox Testing Services is being held this week.
One of the speakers is Michael Bailey, who started abusing alcohol and drugs because of stress at work.
He said he wants to see positive changes that can help people in his position.
“I started using alcohol to cope with the pressures of work and no one looked twice so long as I was doing my job and bringing in sales,” he said.
“The first time the company brought up alcohol procedures was when I’d lost my driving licence and I was fired.
“That was the point when I lost everything - my marriage, my kids and my home.
“It’s taken four years to come back from the brink.
“We’ve got to end the stigma and enable people to ask for help. If employers want to make a positive change, they’ve got to have testing and support services in place.”