Bleeding

• In higher dosages, fish oil can make it hard to stop bleeding. In those who already have a hard time with bleeding issues this can create a very serious problem. Do not start taking fish oil supplements until you have talked to your doctor if you have a problem with this already, as it may make it worse.
  
  

Cholesterol

• Blood tests may be required to test your levels of LDL, the bad cholesterol, if taking fish oil supplements in higher dosages or for longer periods of time, as fish oils can raise LDL levels quickly if not monitored. This is fairly common in those who take fish oil regularly, and shouldn't be a problem unless LDL levels become too high.
  
  

Immune System

• When taking fish oil supplements, it can slow your immune system down and inhibit your body's ability to fight infections. This is especially serious for those who already have weakened or compromised immune systems, or who are taking other medications that also weaken their immune system. Those with conditions that lower immune system defenses, such as HIV/AIDS, should not take fish oil.
  
  

Worsening Conditions

• Those who already have certain conditions shouldn't take fish oil, as it may make your condition or symptoms of it worse. Depression and bipolar disorder can both become worse while taking fish oil supplements, as can diabetes and high blood pressure. If you already have a condition, talk to your doctor or pharmacist before starting a fish oil supplement regimen to make sure you are taking it safely.
  
  

Interactions

• Certain other medications can interact with the effectiveness of fish oil, so those taking birth control pills should not take a fish oil supplement. High blood pressure medications as well as some weight loss supplements may also interact with fish oil. If you have any concerns about the interactions that your current medications or supplements may have with fish oil, then you should talk to your doctor before starting a fish oil regimen.