While a great leader will challenge you to become your best, a toxic boss will drain your mental strength.

As a therapist, I've had many clients enter my office saying things like, "My boss is a bully. I might have to quit my job," and "I used to like my job. But under new leadership, it's intolerable. I'm not sure how much longer I can do this."

In a rural area, it's common to see several people working for the same boss. Of course, none of them know their co-workers are also in treatment struggling to deal with a tyrant for a boss.

But it's clear that working for a toxic boss can take a serious toll on your well-being. And new research shows just how damaging it can be.

What Makes A Boss Toxic

An inept leader isn't necessarily toxic. Some people simply lack basic leadership skills, like communication and organization.

Toxic leaders, however, proactively harm others. They use fear and intimidation to maintain control. They embarrass employees and threaten people as a way to gain compliance.

Toxic bosses frequently exhibit psychopathic and narcissistic traits. They lack empathy for others and they abuse their power.

The Damage Toxic Bosses Can Cause

Researchers at University of Manchester's Business School examined how employees are affected by toxic bosses. They studied 1,200 participants from a variety of industries in several different countries.

Not surprisingly, employees with a toxic boss experienced lower rates of job satisfaction.

But the researchers also found that employees' misery spilled over into their personal lives too. Working for a toxic boss wreaked havoc on their mental health.

Employees working for a narcissistic or psychopathic boss were more likely to experience clinical depression.

Toxic Bosses Damage the Company Culture

The same study also found that a toxic leader's behavior spreads like wildfire in the workplace.

Under toxic leadership, employees are more likely to criticize one another and they're more inclined to take credit for other people's work. They also behave more aggressively toward one another.

Other studies have found similar results. A 2016 study conducted by the University of Michigan found that rude behavior, like sarcasm and put-downs, leads to mental fatigue.

Consequently, workers had less self-control, which increased the chances they'd be rude toward others.

Workplace incivility isn't just bad for morale--it also hurts a company's bottom dollar. Researchers estimate disrespectful behavior costs companies $14,000 per employee due to lost productivity.

Why Companies Should Examine How Leadership Success is Measured

Unfortunately, many organizations measure a leader's success based on short-term performance. But, toxic bosses are often able to create short-term changes that look like improvements.

Threats and intimidation may lead employees to work longer hours and become more productive--initially. But, as a boss' toxic behavior continues, employees lose motivation and productivity declines.

Eventually, employees are at a higher risk for burnout. And the stress will take a toll on their well-being and their performance.

It's important for organizations to look at the bigger picture when examining the success of a leader. A toxic boss will have a ripple effect that lasts for years.