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Brand & Generic Names


Basic Information

  • Habit forming? No
  • Prescription needed? Yes
  • Available as generic? No
  • Drug class: Antihypertensive


    Controls, but doesn't cure, high blood pressure.

Dosage & Usage Information

How to take:
Tablet--Swallow with liquid or food to lessen stomach irritation. If you can't swallow whole, crumble tablet and take with liquid or food.

When to take:
At the same time each day.

If you forget a dose:
Take as soon as you remember up to 2 hours late. If more than 2 hours, wait for next scheduled dose (don't double this dose).

What drug does:
Relaxes muscle cells of small arteries.

Time lapse before drug works:
4 to 6 hours. May need to take for lifetime.

Don't take with:
Any other medicine without consulting your doctor or pharmacist.


Severe blood pressure drop; fainting; blurred vision; slow, weak pulse; cold, sweaty skin; loss of consciousness.


  • Dial 911 (emergency) or O (operator) for an ambulance or medical help. Then give first aid immediately.
  • See EMERGENCY Information.

Possible Adverse Reactions or Side Effects


    In case of overdose, see Overdose section.


  • Diarrhea, more bowel movements, fatigue, weakness.
  • Dizziness, lower sex drive, feet and ankle swelling, drowsiness.
  • Stuffy nose, dry mouth.


  • Rash, blurred vision, drooping eyelids, chest pain or shortness of breath, muscle pain or tremor.
  • Nausea or vomiting, headache.
  • Impotence, nighttime urination.


    Decreased white blood cells causing sore throat, fever.

Warnings & Precautions

Don't take if:

  • You are allergic to guanadrel.
  • You have taken MAO inhibitors within 2 weeks.

Before you start, consult your doctor:

  • If you have stroke or heart disease.
  • If you have asthma.
  • If you have had kidney disease.
  • If you have peptic ulcer or chronic acid indigestion.
  • If you will have surgery within 2 months, including dental surgery, requiring general or spinal anesthesia.

Over age 60:
Adverse reactions and side effects may be more frequent and severe than in younger persons. Start with small doses and monitor blood pressure frequently.

No proven harm to unborn child. Avoid if possible. Consult doctor.

No proven harm to nursing infant. Avoid if possible. Consult doctor.

Infants & children:
Not recommended.

Prolonged use:

  • Due to drug's cumulative effect, dose will require adjustment to prevent wide fluctuations in blood pressure.
  • Talk to your doctor about the need for follow-up medical examinations or laboratory studies.

Skin & sunlight:
No problems expected.

Driving, piloting or hazardous work:
Don't drive or pilot aircraft until you learn how medicine affects you. Don't work around dangerous machinery. Don't climb ladders or work in high places. Danger increases if you drink alcohol or take medicine affecting alertness and reflexes, such as antihistamines, tranquilizers, sedatives, pain medicine, narcotics and mind-altering drugs.

Don't discontinue without consulting doctor. Dose may require gradual reduction if you have taken drug for a long time. Doses of other drugs may also require adjustment.

Hot weather further lowers blood pressure, particularly in patients over 60.

Possible Interaction with Other Drugs

------------------------ -----------------------
ACE inhibitors*: Possible excessive
captopril, enalapril, potassium in blood.
Antidepressants, Decreased effect of
tricyclic (TCA)* guanadrel.
Antihypertensives, Increased effect of
other* guanadrel.
Beta-adrenergic Increased likelihood of
blockers* dizziness and fainting.
Carteolol Increased antihypertensive effect.
Contraceptives*, Increased side effects
oral of oral contraceptives.
Central nervous system Decreased effect of guanadrel.
Diuretics* Increased likelihood of dizziness
and fainting.
Haloperidol Decreased effect of guanadrel.
Insulin Increased insulin effect.
Loxapine Decreased effect of guanadrel.
MAO inhibitors* Severe high blood pressure. Avoid.
Nicardipine Blood pressure drop. Dosages may
require adjustment.
Nimodipine Dangerous blood pressure drop.
Phenothiazines* Decreased effect of guanadrel.
Rauwolfia alkaloids* Increased likelihood of dizziness
and fainting.
Sotalol Increased antihypertensive effect.
Sympathomimetics* Decreased effect of guanadrel.
Terazosin Decreases effectiveness of
Thioxanthenes* Decreased effect of guanadrel.
Trimeprazine Decreased effect of guanadrel.

Possible Interaction with Other Substances

--------------- ---------------
Alcohol: Decreased effect of guanadrel.
Caffeine. Decreased effect of guanadrel.
Cocaine: Increased risk of heart block and
high blood pressure.
Foods: No problems expected.
Marijuana: Higher blood pressure. Avoid.
Tobacco: Higher blood pressure. Avoid.

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