The cause of low blood pressure isn't always clear. It may be associated with the following:

• Pregnancy
  
• Hormonal problems such as an underactive thyroid (hypothyroidism), diabetes, or low blood sugar (hypoglycemia)
  
• Some over-the-counter medications
  
• Some prescription medicines such as for high blood pressure, depression or Parkinson’s disease
  
• Heart failure
  
• Heart arrhythmias (abnormal heart rhythms)
  
• Widening, or dilation, of the blood vessels
  
• Heat exhaustion or heat stroke
  
• Liver disease

Sudden drops in blood pressure can be life-threatening. Causes of this type of hypotension include:

• Loss of blood from bleeding
  
• Low body temperature
  
• High body temperature
  
• Heart muscle disease causing heart failure
  
• Sepsis, a severe blood infection
  
• Severe dehydration from vomiting, diarrhea, or fever
  
• A reaction to medication or alcohol
  
• A severe allergic reaction, called anaphylaxis

Postural hypotension, which is low blood pressure when standing up suddenly, can happen to anyone for a variety of reasons, such as dehydration, lack of food, or being overly fatigued. It can also be influenced by genetic make-up, aging, medication, dietary and psychological factors, and acute triggers, such as infection and allergy.

Postural hypotension occurs most frequently in people who are taking drugs to control high blood pressure (hypertension). It can also be related to pregnancy, strong emotions, hardening of the arteries (atherosclerosis), or diabetes. The elderly are particularly affected, especially those who have high blood pressure or autonomic nervous system dysfunction.

Hypotension after meals is a common cause of dizziness and falls after eating. This is most common after large meals containing a lot of carbohydrates. It’s believed to be caused by blood pooling into the vessels of the stomach and intestines.

Several drugs are commonly associated with postural hypotension. These medications can be divided into two major categories:

• Drugs used to treat high blood pressure, such as diuretics, beta-blockers, calcium-channel blockers, and angiotensin-converting enzyme (ACE) inhibitors
  
• Drugs that have hypotension as a side effect, including nitrates, drugs for Parkinson's disease, antipsychotics, neuroleptics, anti-anxiety agents, sedative-hypnotics, and tricyclic antidepressants

Common causes of naturally occurring postural hypotension include:

• Dehydration and electrolyte loss, which may result from diarrhea, vomiting, excessive blood loss during menstruation, or other conditions
  
• Age-associated decline in blood pressure regulation, which may be worsened by certain health conditions or medications

Certain diseases can also cause postural hypotension. These include:

• Central nervous system disorders, such as Shy-Drager syndrome or multiple system atrophy
  
• Nerve problems, such as peripheral neuropathy or autonomic neuropathy
  
• Cardiovascular disorders
  
• Alcoholism
  
• Nutritional diseases

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