Mar Vista Animal Medical Center

3850 Grand View Blvd.
Los Angeles, CA 90066





25 MG, 50 MG, AND 100 MG


The branch of the nervous system that covers automatic body functions such as breathing, sweating, digestion, and other similar involuntary processes is called the "autonomic" nervous system. (It may be easiest to think of the word "autonomic" to mean "automatic.")

The autonomic nervous system is divided into the sympathetic and parasympathetic nervous systems: the sympathetic system when stimulated prepares the body for "fight or flight" situations while the parasympathetic system when stimulated maintains a "status quo" state.

The fight or flight situation is more or less just what it sounds like: the body gets ready to do what is necessary to save its life. This might involve running from a predator, struggling with an enemy, or simply experiencing fear or anxiety. The actual fighting or fleeing is voluntary but there plenty of involuntary body modifications that make effective fighting or fleeing possible. The pupils dilate, the heart rate increases, blood circulation diverts from the digestive tract and towards the muscles of the legs and arms, etc. The receptors in our different organs that are involved in the sympathetic nervous system are alpha receptors and beta receptors. Atenolol concerns the beta receptors in the heart, which are called "beta-one" receptors.

When the heart's beta-one receptors are stimulated, the heart rate increases and the heart begins to pump with greater strength. This is all well and good in a fight or flight situation but for everyday life, it is stressful to the heart and can lead to unnecessary heart muscle fatigue.

As one might guess, if there are beta-one receptors, there are also "beta-two receptors". When stimulated, beta-two receptors serve to relax the airways of the lung (for deeper breaths), facilitate fat mobilization, thicken salivary secretions, as well as perform other actions. Atenolol is a beta-one blocker and is not involved with the beta-two receptors. Atenolol blocks the function of only the beta-one receptors.

Normal Heart
(original graphic by



Certain heart diseases, such as subaortic stenosis of dogs, and hypertrophic cardiomyopathy of cats, involve the development of thickened, inflexible heart muscle. These heart diseases can benefit from what is called "beta blockade." This means that atenolol can relax the stiff heart muscle and slow the heart rate, allowing for more efficient filling of the heart chambers.

Atenolol might also be used in the treatment of high blood pressure or to treat heart arrhythmias where the heart rhythm is simply too fast (and the heart does not have time to properly fill between beats).

Sometimes atenolol is used in preparing feline patients for surgical treatment of hyperthyroidism, where there is often heart disease as described.



Atenolol has a tendency to drop blood pressure through its effects on the heart. This may or may not be a desired effect. Low blood pressure in a patient may manifest as lethargy/listlessness. If this side effect is observed it may be well worth having the patient's blood pressure checked. Listlessness from low blood pressure would be noted as soon as within an hour of oral administration. Atenolol slows the heart rate and can weaken contraction of the heart; these may be harmful depending on what type of heart disease is present. These effects, for example, could make heart failure worse.

Some patients develop diarrhea from taking atenolol. If this or any other side effect becomes problematic, it is recommended to withdraw the atenolol gradually rather than suddenly.

Other beta blockers have created problems facilitating airway constriction in asthmatic patients but atenolol, being specific to beta-one receptors at usual doses, does not have this problem.



Drugs that promote the function of the sympathetic nervous system may be blocked by atenolol and may in turn block the effectiveness of atenolol. Such drugs include: phenylpropanolamine (used for urinary incontinence) and terbutaline (an airway dilator).

Combining atenolol with acepromazine (a tranquilizer), furosemide (a diuretic), or hydralazine (a blood vessel dilator) may enhance the low blood pressure side effect of atenolol. Combining atenolol with heart medications called "calcium channel blockers" may enhance the heart rate slowing effects of atenolol.



  • Atenolol should not be used in patients in heart failure. These patients need their hearts to be beating with appropriate strength and speed and can destabilize if this is disrupted.
  • Atenolol can disrupt blood sugar control in diabetic patients.
  • Atenolol should be stored at room temperature and should be protected from light.
  • Atenolol should be avoided in patients with kidney failure.

 Page posted: 1/10/2013
Page last updated: 5/4/2017