Balance

• Working a second job can rob you of your free time, time you could devote to achieving a work-life balance. Spending eight hours at your primary job and going from there to a second job, or working weekends on a second job limits the time you can spend with family, friends or simply time to yourself for rejuvenation and relaxation.
  
  

Focus

• One of the disadvantages of a second job is that it may steer you away from your primary focus -- your career. If you are working a second job that requires even a fraction of the amount of mental capacity that your profession requires, you could be diverting your attention too much away from your professional career and aspirations. For example, if you work for an accounting firm in a career track position, yet you spend your nights and weekends teaching accounting courses at the local college, you may be expending energy that may be better spent focusing on the succession plan designed for you at your workplace.
  
  

Worth

• Calculating your worth means more than just establishing a marketplace value. The time and energy you spend working may not be worth the extra money you make from a second job. If your second job requires additional expenses just to maintain employment, such as uniforms, travel, child care expenses and incidentals, it's questionable whether your net gain from a second job is really worthwhile.
  
  

Policy

• Some employers have policies that prohibit moonlighting. For example, H&R Block, a name well-known for its tax preparation services, prohibits employees from taking on side work preparing taxes. Workplace policies that prohibit moonlighting eliminate potential conflicts of interest, and protect your primary employer's interests by preventing you from poaching the firm's clients.
  
  

Taxes

• If your current income is on the edge of one tax bracket, income from a second job could push you into another tax bracket, which could significantly increase your tax liability. Your income from the second job may not be enough to justify the potential of the added tax liability.
  
  

Exhaustion

• A second job that involves manual labor could be physically and mentally taxing. Exhaustion and tiredness can affect your ability to concentrate on your primary job, increase your risk of injury and change your disposition, making you irritable and difficult to work with.