Hangovers are causing 11.5 million "sick days" a year at a cost of $3 billion to the Australian economy, new research suggests.
There are also fears that people who are mixing alcohol with amphetamines on the weekends are experiencing "Weepy Wednesdays" because of the delayed effects of their drug use, making them irritable and unreliable workers. 
A Flinders University study has found that the more alcohol and/or drugs an employee consumes, the more time they are likely to take off work. 
After tallying the total cost of this lost productivity at $3 billion a year - up from $1.2 billion in alcohol-related absenteeism alone in 2001 - the researchers said employers should be looking at ways to promote a healthier culture in their workplace especially around alcohol, which is by far the biggest problem.
Researchers at the National Centre for Education and Training on Addiction looked at data from the 2013 National Drug Strategy Household Survey, which asked more than 12,000 people about their habits and how often they missed work, university or school because of them.
While most (56 per cent) drank alcohol at low risk levels (four or less drinks on one occasion), 27 per cent drank at risky levels (five to 10 drinks in one session) and 9 per cent drank at a high risk level (more than 11 drinks in one stint).
When asked about illicit drug use, 7 per cent used drugs yearly, 3 per cent did so monthly and 5 per cent used weekly.
Using the participants' responses, the researchers estimated Australians missed a total of 1.6 million days due to alcohol and about 854,000 days due to drug use that year, with rates of absence increasing with riskier and more frequent consumption.
The researchers then used two measures to work out the cost. The first measure multiplied the self-reported number of days missed by $267.70 (one day's wage  plus 20 per cent employer on-costs).
The second measure calculated the amount of any illness/injury absenteeism attributable to alcohol and drug use by estimating the average difference in absence for those who used alcohol or drugs compared to abstainers. This figure was also multiplied by $267.70.
On the first measure, 2.5 million days were lost annually, costing $680 million. On the second, 11.5 million days were lost annually, costing $3 billion.
Lead author of the study, Ann Roche, said people indulging in alcohol and drugs on the weekend may not realise it is causing their stomach upset, headache or worsening cold by Monday, making the second measure arguably more reliable. 
"Alcohol puts a bit of a tax on your immune system ??? and it's quite implicated in mental health problems. If people are prone towards anxiety and depression, they often self medicate with alcohol," she said.  
Professor Roche said there was also anecdotal evidence of "Weepy Wednesdays" among people who use amphetamines and alcohol on the weekend and then experience a deterioration of their general wellbeing during the week.
While the average amount of alcohol being consumed by Australians has been decreasing, Professor Roche said there was evidence a minority of people were drinking extreme amounts of alcohol - enough to end up in hospital.
"There has also been a statistically significant increase in baby boomers drinking at risky levels. We've never seen that before," she said.
Professor Roche said Australian businesses should promote healthy behaviour to reduce the costs of absenteeism. She recommended formal alcohol and drug policies, education and training for staff and confidential access to counselling and treatment if people need it.  
"The good news is that these kinds of strategies have been shown to be highly effective," she said.