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Medications Lexicon

How does this medication work? What will it do for me?

This is a combination product that contains 2 medications: atenolol and chlorthalidone. It is used to treat high blood pressure. Atenolol belongs to the class of medications called beta-blockers. It works by decreasing the workload of the heart. Chlorthalidone belongs to the class of medications called diuretics (water pills). It works by getting rid of excess salt and water.

Your doctor may have suggested this medication for conditions other than the ones listed in these drug information articles. As well, some forms of this medication may not be used for all of the conditions discussed here. If you have not discussed this with your doctor or are not sure why you are taking this medication, speak to your doctor. Do not stop taking this medication without consulting your doctor.

Do not give this medication to anyone else, even if they have the same symptoms as you do. It can be harmful for people to take this medication if their doctor has not prescribed it.

How should I use this medication?

The eventual dose of atenolol-chlorthalidone is determined by treating with each of the ingredients separately (i.e., atenolol and chlorthalidone) and finding the best daily dose for each. The combination medication may then be used at the appropriate dose.

The recommended dose of atenolol-chlorthalidone ranges from 1 tablet of the 50 mg/25 mg combination daily to 1 tablet of the 100 mg/25 mg combination daily. If blood pressure is not adequately controlled after using the higher strength medication, your doctor may prescribe an additional blood pressure-lowering medication that works in a different way.

Many things can affect the dose of medication that a person needs, such as body weight, other medical conditions, and other medications. If your doctor has recommended a dose different from the ones listed here, do not change the way that you are taking the medication without consulting your doctor.

It is important to take this medication exactly as prescribed by your doctor. If you miss a dose, take it as soon as possible and continue on with your regular schedule. If it is within 8 hours of your next dose, skip the missed dose and continue on with your regular dosing schedule. Do not take a double dose to make up for a missed one. If you are not sure what to do after missing a dose, contact your doctor or pharmacist for advice.

Do not stop taking this medication suddenly without checking with your doctor first. Stopping this medication suddenly after you have been taking it for a while may cause unpleasant and potentially harmful side effects.

Store this medication at room temperature, protect it from light and moisture, and keep it out of the reach of children.

Do not dispose of medications in wastewater (e.g. down the sink or in the toilet) or in household garbage. Ask your pharmacist how to dispose of medications that are no longer needed or have expired.

What form(s) does this medication come in?

50 mg/25 mg
Each white, round, biconvex tablet, scored and identified "50" over "25" on one side and "APO" on the other, contains atenolol 50 mg and chlorthalidone 25 mg. Nonmedicinal ingredients: colloidal silicon dioxide, crospovidone, magnesium stearate, and microcrystalline cellulose.

100 mg/25 mg
Each white, round, biconvex tablet, scored and identified "100" over "25" on one side and "APO" on the other, contains atenolol 100 mg and chlorthalidone 25 mg. Nonmedicinal ingredients: colloidal silicon dioxide, crospovidone, magnesium stearate, and microcrystalline cellulose.

Who should NOT take this medication?

Do not use atenolol - chlorthalidone if you:

  • are allergic to atenolol, chlorthalidone or any ingredients of the medication
  • are allergic to sulfa medications (e.g. sulfamethoxazole)
  • are not able to expel urine
  • are pregnant or breast-feeding
  • are receiving certain anesthetics before surgery
  • have a condition called cardiogenic shock
  • have a condition known as pheochromocytoma (in the absence of alpha-blockade)
  • have a condition known as right ventricular failure caused by pulmonary hypertension
  • have a severely slow heart rate
  • have a serious heart block, also called atrioventricular (AV) block
  • have metabolic acidosis
  • have severe peripheral arterial disorders (blockage of blood flow to limbs of body)
  • have sick sinus syndrome
  • have uncontrolled congestive heart failure
  • have very low blood pressure

What side effects are possible with this medication?

Many medications can cause side effects. A side effect is an unwanted response to a medication when it is taken in normal doses. Side effects can be mild or severe, temporary or permanent.

The side effects listed below are not experienced by everyone who takes this medication. If you are concerned about side effects, discuss the risks and benefits of this medication with your doctor.

The following side effects have been reported by at least 1% of people taking this medication. Many of these side effects can be managed, and some may go away on their own over time.

Contact your doctor if you experience these side effects and they are severe or bothersome. Your pharmacist may be able to advise you on managing side effects.

  • anxiety or nervousness
  • constipation
  • decreased sexual ability
  • diarrhea
  • dizziness or lightheadedness
  • drowsiness (mild)
  • fatigue
  • increased sensitivity of skin to sunlight (skin rash, itching, redness or other discolouration of skin, or severe sunburn)
  • loss of appetite
  • nausea
  • numbness or tingling in the fingers and toes
  • stomach discomfort or upset
  • stuffy nose
  • trouble sleeping

Although most of the side effects listed below don't happen very often, they could lead to serious problems if you do not seek medical attention.

Check with your doctor as soon as possible if any of the following side effects occur:

  • breathing difficulty or wheezing
  • chest pain
  • cold hands and feet
  • depression
  • fever, chills, cough, or sore throat
  • hallucinations
  • joint pain
  • lower back or side pain
  • pinpoint red spots on the skin
  • red, scaling, or crusted skin
  • skin rash or hives
  • slow heartbeat (especially less than 50 beats per minute)
  • swelling of the ankles, feet, or lower legs
  • stomach pain (severe) with nausea and vomiting
  • unusual bleeding or bruising
  • yellow eyes or skin

Check with your doctor as soon as possible if any of the following signs of too much potassium or sodium loss occur:

  • confusion
  • convulsions
  • dry mouth
  • increased thirst
  • irregular heartbeat
  • irritability
  • mood or mental changes
  • muscle cramps or pain
  • nausea or vomiting
  • unusual tiredness or weakness
  • weak pulse

Check with your doctor as soon as possible if any of the following signs of overdose occur:

  • bluish-coloured fingernails or palms of the hands
  • convulsions
  • decreased blood pressure
  • difficulty in breathing
  • dizziness (severe) or fainting
  • hypoglycemia
  • slow heartbeat

Some people may experience side effects other than those listed. Check with your doctor if you notice any symptom that worries you while you are taking this medication.

Are there any other precautions or warnings for this medication?

Before you begin using a medication, be sure to inform your doctor of any medical conditions or allergies you may have, any medications you are taking, whether you are pregnant or breast-feeding, and any other significant facts about your health. These factors may affect how you should use this medication.

Breathing conditions: If you have asthma and certain other breathing problems, you should not, in general, take beta-blockers such as atenolol. Low doses of atenolol may be taken by people with asthma who do not respond to, or cannot tolerate, alternative treatment but they should be monitored by their doctor.

Cholesterol levels: Increases in cholesterol and triglyceride levels may occur when taking chlorthalidone. Talk to your doctor about whether you need to have your cholesterol levels tested.

Diabetes: Chlorthalidone may make it more difficult for people with diabetes to control their blood sugar. The signs of low blood sugar may not be as noticeable when taking atenolol. Dose adjustment of diabetes medications, including insulin, may be required. If you have diabetes and take insulin or other medications that work by affecting the sugar in the blood, you should monitor your blood sugar carefully while taking this medication.

Drowsiness/reduced alertness: Dizziness or fatigue may occur when starting treatment with this medication. This may impair your ability to drive or operate machinery. Exercise caution with these activities until you find out whether the medication affects you in this way.

Fluid and electrolyte balance: The levels of electrolytes such as potassium, sodium, magnesium, and chloride can be reduced by the use of chlorthalidone. The doctor will periodically check to see whether these are in balance. A potassium supplement may be necessary when taking this medication.

Gout: High levels of uric acid may occur in the blood or gout may be brought on in certain people receiving chlorthalidone.

History of heart failure: If you have a history of heart failure, discuss with your doctor how this medication may affect your medical condition, how your medical condition may affect the dosing and effectiveness of this medication, and whether any special monitoring is needed.

Hyperthyroidism (high level of thyroid hormones): People with hyperthyroidism should be monitored by their doctor while taking this medication as it may reduce the symptoms of this condition giving a false impression of improvement.

Kidney disease: People with kidney disease should be monitored by their doctor while taking this medication as they are at increased risk of experiencing side effects.

Liver function: People with reduced liver function or progressive liver disease should be monitored by their doctor while taking this medication.

Prinzmetal's angina: Atenolol may increase the number and duration of angina attacks in people with Prinzmetal's angina. If you have this condition, discuss with your doctor how this medication may affect your medical condition, how your medical condition may affect the dosing and effectiveness of this medication, and whether any special monitoring is needed.

Severe allergies: If you have allergies severe enough to cause anaphylaxis (a severe allergic reaction where swelling of the face, lips, and throat make it difficult to breathe), discuss with your doctor about what to do if you have an allergic reaction. Atenolol may make it more difficult to treat your allergic reaction with epinephrine.

Stopping usage: Atenolol should not be stopped suddenly if you have angina. There have been reports of severe worsening of angina and of heart attack or abnormal heart rhythms occurring in people with angina pectoris who have done this.

Surgery: If you are scheduled for surgery, inform all doctors involved in your care that you are taking a medication that contains atenolol.

Systemic lupus erythematosus: People with systemic lupus erythematosus should be monitored by their doctor while taking chlorthalidone as it may worsen this condition.

Pregnancy: This medication should not be used during pregnancy. If you become pregnant while taking this medication, contact your doctor immediately.

Breast-feeding: Atenolol and chlorthalidone both pass into breast milk. Women who take this medication should not breast-feed.

Children: The safety and effectiveness of using this medication have not been established for children. It is not recommended for use by children.

What other drugs could interact with this medication?

There may be an interaction between atenolol - chlorthalidone and any of the following:

  • alcohol
  • amlodipine
  • anesthetic agents
  • anticholinergics (e.g., atropine, hyoscyamine)
  • asthma medications (e.g., theophylline)
  • barbiturates (e.g., phenobarbital)
  • clonidine
  • diabetes medications (e.g., insulin)
  • digoxin
  • diltiazem
  • disopyramide
  • epinephrine
  • felodipine
  • flecainide
  • guanethidine
  • lidocaine
  • lithium
  • methacholine
  • mexiletene
  • monoamine oxidase (MAO) inhibitors (e.g., tranylcypromine, phenelzine) taken within the past 2 weeks
  • narcotic analgesics (e.g., morphine, codeine)
  • nifedipine
  • nonsteroidal anti-inflammatory drugs (e.g., ibuprofen)
  • norepinephrine
  • other beta-blockers (e.g., propranolol, metoprolol)
  • procainamide
  • propafenone
  • quinidine
  • reserpine
  • salicylates (e.g., ASA, salsalate)
  • tubocurarine
  • verapamil

If you are taking any medications containing this drug, speak with your doctor or pharmacist. Depending on your specific circumstances, your doctor may want you to:

  • stop taking one of the medications,
  • change one of the medications to another,
  • change how you are taking one or both of the medications, or
  • leave everything as is.

An interaction between two medications does not always mean that you must stop taking one of them. Speak to your doctor about how any drug interactions are being managed or should be managed.

Medications other than those listed above may interact with this medication. Tell your doctor or prescriber about all prescription, over-the-counter (non-prescription) and herbal medications that you are taking. Also tell them about any supplements you take. Since caffeine, alcohol, the nicotine from cigarettes, or illegal drugs can affect the action of many medications, you should let your prescriber know if you use them.

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