Patients should consult their physician before taking any other medication (either prescription or over-the-counter), nutritional supplements or herbal remedies. Substances that can intensify the effects of antihypertensives and possibly lead to hypotension (low blood pressure) include:

• Other antihypertensives (unless prescribed as part of a combination therapy).
  
  
• Nitrates (medications that relax the arterial walls, causing a drop in blood pressure).
  
  
• Alcohol.
  
  
• Grapefruit juice. Grapefruit juice interferes with the liver’s ability to rid the body of some substances. This could lead to a buildup of antihypertensives to toxic levels in the body. Though the buildup is less likely if the juice is ingested four or more hours before taking the medicine, patients taking antihypertensives are advised to refrain from drinking grapefruit juice. There is no indication that eating grapefruit (as opposed to drinking juice) presents any danger to patients.
  
  
• Pomegranate juice. This fruit juice may reduce blood pressure. Patients taking antihypertensives are advised to ask their physician whether they can safely drink pomegranate juice and whether doing so can affect their dosage.
  
  
• Certain other foods. Some research suggests that foods including dark chocolate and soy protein may lower blood pressure. 

The following substances may counteract the effects of some antihypertensives and increase blood pressure.

• Opioids (prescription narcotic painkillers).
  
  
• Nicotine (present in tobacco products such as cigarettes).
  
  
• Nonsteroidal anti-inflammatory drugs (NSAIDs) such as aspirin, ibuprofen, naproxen and COX-2 inhibitors. The U.S. Food and Drug Administration (FDA) warned in 2005 that COX-2 inhibitors may cause an increase in serious gastrointestinal bleeding and cardiovascular events such as heart attack and stroke. Some of the medications have been removed from the market. Because of these issues, the side effects caused by NSAIDs and their many interactions with other drugs and substances, patients are encouraged to discuss NSAIDs with their physician before using the drugs.

In addition, some over-the-counter medications may adversely react with antihypertensives, including diet pills and medications for asthma, colds, flu, cough, sinus problems or hay fever. People should also be careful of using eye drops for bloodshot eyes.