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Brand & Generic Names

Apo-Guanethidine Ismelin

Basic Information

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


    Reduces high blood pressure.

Dosage & Usage Information

How to take:
Tablet--Swallow with liquid. If you can't swallow tablet whole, crumble 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:
Displaces norepinephrine--hormone necessary to maintain small blood vessel tone. Blood vessels relax and high blood pressure drops.

Time lapse before drug works:
Regular use for several weeks may be necessary to determine effectiveness.

Don't take with:
Non-prescription drugs containing alcohol without consulting doctor.
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.


  • Unusually slow heartbeat.
  • Diarrhea; more bowel movements; swollen feet, legs; fatigue, weakness.
  • Dizziness, lower sex drive.
  • 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, hair loss.


    Decreased white blood cells causing sore throat, fever.

Warnings & Precautions

Don't take if:

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

Before you start, consult your doctor:

  • If you have had 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.

Studies inconclusive on harm to unborn child. Decide with you doctor whether drug benefits justify risk to unborn child.

Unknown effect. 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.

Possible Interaction with Other Drugs

------------------------ -----------------------
ACE inhibitors*: Possible excessive
captopril, enalapril, potassium in blood.
Amphetamines* Decreased guanethidine effect.
Antidepressants, Decreased guanethidine
tricyclic (TCA)* effect.
Antidiabetics* Increased guanethidine effect.
Antihistamines* Decreased guanethidine effect.
Carteolol Increased antihypertensive effect.
Contraceptives*, Decreased guanethidine
oral effect. Increased side effects
of oral contraceptives.
Digitalis Slower heartbeat.
Diuretics, thiazide* Increased guanethidine effect.
Haloperidol Decreased guanethidine effect.
Indapamide Possible increased effects of both
drugs. When monitored carefully,
combination may be beneficial in
controlling hypertension.
Insulin Increased insulin effect.
Loxapine Decreased effect of guanethidine.
MAO inhibitors* Increased blood pressure.
Minoxidil Dosage adjustments may be necessary
to keep blood pressure at proper
Nicardipine Blood pressure drop. Dosages may
require adjustment.
Nimodipine Dangerous blood pressure drop.
Phenothiazines* Decreased guanethidine effect.
Rauwolfia Excessively slow
alkaloids* heartbeat. Weakness and faintness
upon rising from chair or bed.
Sotalol Increased antihypertensive effect.
Terazosin Decreases effectiveness of
Thioxanthenes* Decreased guanethidine effect.
Trimeprazine Decreased guanethidine effect.

Possible Interaction with Other Substances

--------------- ---------------
Alcohol: Use caution. Decreases blood
Carbonated drinks. Use sparingly. Sodium content
increases blood pressure.
Cocaine: Increased risk of heart block and
high blood pressure.
Spicy or Avoid if subject to
acid foods. indigestion or peptic ulcer.
Marijuana: Excessively low blood pressure.
Tobacco: Possible blood pressure rise.

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