THE PET PHARMACY By Wendy C. Brooks, DVM, DipABVP Educational Director, VeterinaryPartner.com

(for veterinary use only)

Brand Name: Norvasc

Available in 2.5 mg, 5 mg and 10 mg tablets

Background

Muscle is ultimately composed of protein filaments. These filaments are arranged in parallel in an overlapping fashion. When muscles contract, the fibers are able to slide across one another, shortening the overall length to up to one third of the original length.

Microscopic view of a cross section of skeletal muscle.

Muscle contraction is made possible by the release of calcium from storage within the muscle cell in a structure called the sarcoplasmic reticulum. Nerve stimulation causes calcium to be released, which allows muscle contraction to happen. No calcium, no muscle contraction.

There are three types of muscle within the body: skeletal muscle (the muscles under voluntary control that we use to move), smooth muscle (the involuntary muscle that provides muscle tone in our intestines, blood vessels, and other structures we do not consciously control), and heart muscle (which has some characteristics of both the other two types). The heart must pump against the muscle tone of the arteries and must work harder if there is high pressure in the arteries. When controlling hypertension, the goal is to reduce blood pressure. We would like to relax the muscle tone in the arteries that carry blood to our tissues. Relaxing arterial muscle serves to dilate the artery, which in turn lowers blood pressure (similar to the way water pressure would be reduced by running the same amount of water through a much larger diameter pipe).

Amlodipine besylate is what is called a calcium channel blocker. These drugs work by blocking the calcium needed for muscle contraction in either primarily heart muscle or primarily arterial muscle. Amlodipine besylate is one of the calcium channel blockers that works primarily on arterial muscle. Its overall effect is to relax the arterial muscles so that they dilate and the blood pressure within them drops.

Hypertension is an important problem in both people and pets. Amlodipine besylate has become the most popular blood pressure medication for hypertensive cats. It has not worked well in dogs.

How this Medication Is Used

Amlodipine besylate is used to treat high blood pressure in cats. High blood pressure is a common consequence of numerous conditions, including kidney failure, hyperthyroidism, hypertrophic cardiomyopathy, and others.

Treatment for hypertension should be considered when a cat’s systolic blood pressure exceeds 160 mmHg. Blood pressures over 180 mmHg are considered high risk for causing organ damage.

Amlodipine besylate may be given with or without food. Its peak activity occurs approximately 6 to 9 hours after oral administration in humans (feline data is not available). Amlodipine besylate is generally given once a day to cats. Expect your veterinarian to recommend periodic rechecks to measure blood pressure and evaluate the cat’s retinas for signs of bleeding.

Side Effects

In humans, 7.3% of people taking this drug report headaches, which make headache the most common side effect of amlodipine besylate. Of course, we don’t have a good way of detecting headaches in our pets.

Infrequent side effects reported in cats include elevations in renal blood parameters, drops in blood potassium levels, lethargy, increases in heart rate, and weight loss.

Gingival hyperplasia is a condition where the gums of the mouth overgrow. creating extra space for periodontal bacteria and infection.  This side effect has been reported in 8.5% of dogs on amlodipine. The condition, when it occurs, resolves within 6 months of discontinuation of the drug.

Interactions with Other Drugs

Hypotension (blood pressure dropping too low) generally does not occur with amlodipine besylate unless it is combined with another drug that drops blood pressure. Other drugs of this type might include: fentanyl, diuretics such as furosemide, ACE inhibitors such as enalapril, or beta-blockers such as propranolol.

Concerns and Cautions

Missing even one dose can lead to a significant rise in blood pressure and a return of clinical symptoms of high blood pressure.

• Patients with liver disease remove this drug from their systems very slowly, thus necessitating changes in dose.
• Amlodipine besylate should not be used during pregnancy.

Amlodipine besylate has some (usually insignificant) effects on the heart. It will slightly reduce the strength of contraction. This could be significant in patients with a history of heart failure, and the drug should be used with caution in this situation.

The doses of amlodipine besylate for cats are very small (usually quarter tablets) and it is difficult to cut tablets in appropriate sizes. A pill cutter can be purchased to assist in accurately cutting pills or a compounding pharmacy can be used to formulate custom-made capsules or flavored liquid.

It is our policy not to give dosing information over the Internet.

Date Published: 8/7/2002 12:07:00 AM
Date Reviewed/Revised: 05/01/2009