Brand Name: Proin

Available in 25mg, 50mg, and 75mg chewables; 18mg, 38mg, 74mg, and 145mg extended-release tablets

Background

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 when they happen, 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. In contrast, the sympathetic system initiates changes that are adaptive in times of stress (the so-called fight or 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. Its appetite suppression side effect leads 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 The drug was withdrawn from the human market, and restrictions have been placed on quantities of the veterinary product that can be ordered.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 used in the treatment of urinary incontinence. Regular phenylpropanolamine is generally given twice daily, while the extended-release version may be given once daily.

Only occasionally is this medication used as a decongestant in animals. If a dose is accidentally skipped, do not double up on the next dose. Simply give the medication when it is remembered and time the next dose according to the instructions.

An extended-release tablet that is given once a day has been recently approved.

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.

An increased risk of hypertension can also occur if phenylpropanolamine is given in conjunction with tricyclic antidepressants (such as amitriptyline or clomipramine), non-steroidal anti-inflammatory drugs (NSAIDs), or amitraz (the active ingredient of several tick control products).

Side Effects

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. An increase in 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. Other side effects generally not considered to be serious include a mild upset stomach (generally controlled by giving the medication with food), dilated pupils, panting, and increased thirst.

Potentially serious side effects would include high blood pressure (hypertension).  This is rarely an issue in pets but potentially could happen. Urinary protein loss has also been reported.

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 the effect can be assessed.

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 whether this occurs in dogs and cats.

The extended-release product is not approved for dogs weighing less than 10 lbs.

Regular phenylpropanolamine tablets are given twice daily. Extended-release tablets are given once daily. Extended-release tablets do not come in a size appropriate for dogs under 10 lbs. Extended-release tablets are not meant to be cut or split.

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