Can I sue my co-worker if he contributed to my injury in a workers' compensation case?

If a co-worker acted intentionally and violated company policy (i.e., drug or alcohol use on the job), or started a fight or committed a crime, you might be able to sue a co-worker (click here for more information on third party lawsuits in the workers' compensation context).

How does workers' compensation apply to lawsuits against co-workers?

In all states, workers' comp is designed to take the place of almost all civil law suits. This makes workplace insurance available at fairly reasonable costs by limiting court costs for work-related personal injuries. This also means some actions that would undoubtedly otherwise make someone liable for a lawsuit, cannot be brought, because workers comp is made the only remedy for such injuries. The determining factor is whether a co-worker's alleged actions are related to the workplace.

Doesn't this system protect, or even encourage, claims against co-workers?

Workers compensation does sometimes allow successful claims against co-workers. For example, if there has been an illegal act, it makes it more likely that the right to sue outside of a valid workers' comp claim will be preserved.

What factors go into a decision allowing a lawsuit against a co-worker?

There are enormous differences from state to state. Many states view the actions of a co-worker whose actions are criminal to be acting outside the scope of employment, thus allowing a lawsuit against him/her. Under this reasoning, it would make no sense to limit the right to sue for a workplace injury, since the workplace was not essential to the cause of the injury. Some courts (e.g. - Kentucky) first determine whether the co-worker was acting as an "agent" of the employer. If the co-worker is determined not to be an agent, then their alleged wrongdoing would not be protected by workers' comp statutes.

What is the key fact allowing a suit against co-workers?

In addition to the factors mentioned above, the most important factor tends to be the intention of the the co-worker. If the co-worker intended to commit the wrongdoing against you, then you are much more likely to be allowed an independent lawsuit against him/her (bypassing worker's comp). 

What is a "third party" lawsuit?

Workers comp usually involves two parties: the employer and the employee. Anyone else who may be named in a claim arising from a workplace injury is considered a third party. This is the area of the law in which co-workers are typically sued. Notably, any award in a third party civil suit may also be subject to a lien from a valid workers' comp claim.