By Wendy C. Brooks, DVM, DipABVP
Educational Director, VeterinaryPartner.com
Brand Name: Proin
Available in 25 mg, 50 mg, and 75 mg chewables; 18mg, 38mg, 74mg, and 145mg extended release tablets
In order to understand how phenylpropanolamine works in the body, it is important to understand some background regarding the autonomic nervous system. The autonomic nervous system can be thought of as the “automatic” nervous system in that it controls physiologic functions that one is not aware of. Examples include sweating during times of anxiety, increases and decreases in heart rate or respiratory rate, dilation or constriction of the pupils, blood pressure changes and other functions that enable us to adapt to our changing environment as we perceive it. Our nervous system controls all these things yet we are not consciously aware of any of them happening thanks to the autonomic nervous system.
The autonomic nervous system is divided into the sympathetic and the parasympathetic portions. The easiest way to think of these divisions is that the parasympathetic system maintains the status quo of the body while the sympathetic system initiates changes that are adaptive in times of stress (the so-called fight of flight response.)
The sympathetic nervous system is where phenylpropanolamine acts as a stimulant promoting the fight or flight reflexes within the body. This means that phenylpropanolamine has many effects and thus many uses in treating disease. Its relative safety and efficacy made it a common over-the-counter decongestant for human use once upon a time, while its appetite suppression side effect lead to its wide employment as a human dieting/weight loss aid.
It was accessible in numerous forms on the shelves of every drug store in America but two problems changed all that. The first problem is that this drug was found to increase the incidence of strokes and cerebral hemorrhage in people aged 18 to 49. The second problem is that phenylpropanolamine can be used in the illegal production of methamphetamine. The drug was withdrawn from the human market and restrictions have been placed on quantities of the veterinary product that can be ordered. In some states it is considered a controlled substance.
In veterinary medicine, phenylpropanolamine is used almost exclusively for the control of urinary incontinence in dogs and occasionally in cats. Phenylpropanolamine is able to increase sphincter tone in the urethra thus curtailing inadvertent urine leakage. The increase in high blood pressure that was problematic in humans is not considered a significant issue to the pet population.
How This Medication Is Used
Phenylpropanolamine is generally used 2 to 3 times daily for control of urinary incontinence. An extended release tablet that is given once a day has been recently approved.
If a dose is skipped accidentally, do no double up on the next dose; just give the next dose as you normally would.
Interactions With Other Drugs
In some cases of urinary incontinence, phenylpropanolamine is used in combination with estrogens, such as diethylstilbestrol or estriol. No harmful drug interactions are expected with this combination. In fact, they synergize for a stronger effect.
Phenylpropanolamine should not be used with L-Deprenyl (Anipryl) due to resulting unpredictable fluctuations in blood pressure. It is recommended that phenylpropanolamine be withdrawn for 2 weeks preceding the use of L-Deprenyl.
An increased risk of hypertension can also occur if phenylpropanolamine is given in conjunction with tricyclic antidepressants (such as amitriptyline), non-steroidal anti-inflammatory drugs (NSAIDs), or amitraz (active ingredient of several tick control products).
Phenylpropanolamine stimulates a fight or flight response. This means that the following effects may be observed: rapid heart rate, elevation in blood pressure, and restlessness. Appetite loss or reduction may be a problem. Increased thirst is also a common side effect.
Irritability and restlessness are documented side effects that can occur in humans. It is reasonable to consider that this medication may create similar effects in our pets.
Concerns And Cautions
When initiating therapy with phenylpropanolamine, it is important not to expect an immediate change in urinary incontinence. Several days of proper dosing will be needed before effect can be assessed.
Before using phenylpropanolamine to control urinary incontinence, it is important to rule out other medical causes of incontinence such as kidney disease and bladder infection.
These latter conditions are progressive and should be identified early in their course for meaningful treatment results.
Phenylpropanolamine should be stored in containers that protect it from light. Light exposure leads it to lose potency.
Phenylpropanolamine acts by causing the release of a hormone and neurotransmitter called norepinephrine. With chronic use, it is possible to deplete the body’s stores of norepinephrine and the patient will appear to become resistant to the effects of the drug. This phenomenon is well described in people who use phenylpropanolamine as a decongestant but it is unclear as to whether this occurs in dogs and cats.
The extended release product is not approved for dogs weighing less than 10 lbs.
Because of its effects in elevating heart rate and blood pressure, phenylpropanolamine should not be used in patients with heart disease or pre-existing high blood pressure. This includes patients with glaucoma, hyperthyroidism, and diabetes mellitus as well as those with certain types of cardiovascular disease. Check with your veterinarian if there is any question.
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