Pregnancy

• Unexplained nausea is an early indicator of pregnancy (known as morning sickness, though it occurs at any time). Typically, it begins around the sixth week and ends by the third month.
  
  

Medication

• If you take any prescription or over-the-counter medication, check whether it can cause nausea as a side effect.
  
  

Infection

• Many types of infections, particularly in the gastrointestinal tract, can cause unexplained nausea.
  
  

Food Poisoning

• Nausea can result from eating food contaminated with bacteria. Usually this will be followed by vomiting and/or diarrhea.
  
  

Stress/Psychiatric Disorders

• Stress, anxiety, depression and many mood disorders can manifest physically as unexplained nausea.