Overactive bladder (OAB) is a condition defined by a frequent and sudden, compelling need to urinate. Among the modern therapies for this condition are beta-3 agonists, a class of medication offering a targeted approach to controlling OAB symptoms. These drugs work differently from older treatments by focusing on a specific pathway to help the bladder function more normally during its storage phase.
Mechanism of Action in the Bladder
The function of beta-3 agonists is centered on their interaction with specific receptors located within the bladder wall. The bladder’s main muscle, the detrusor, is rich in beta-3 adrenergic receptors. When a beta-3 agonist binds to and activates these receptors, it initiates a chemical signaling process that causes the detrusor muscle to relax.
This induced relaxation counteracts the involuntary muscle contractions characteristic of OAB. By calming the detrusor muscle, the medication allows the bladder to hold a larger volume of urine before signaling the brain that it is full. This increase in bladder capacity directly addresses the primary symptoms of OAB, reducing urinary frequency and feelings of urgency.
Further research suggests the mechanism may be more complex than direct muscle relaxation alone. Activation of the beta-3 adrenergic receptor may also involve the release of other substances that contribute to muscle relaxation. Some studies propose these drugs might also influence nerve signals, potentially decreasing the sensory nerve activity that contributes to the feeling of urgency. This dual action on the muscle and its nerve signaling pathways helps to normalize bladder function.
Common Beta-3 Agonist Prescriptions
Two primary beta-3 agonist medications are commonly prescribed for overactive bladder. These drugs are administered as oral tablets for once-daily use, providing a convenient treatment regimen.
The first medication in this class to receive FDA approval was mirabegron (Myrbetriq), which is available in tablet form or as granules for an oral suspension. Another widely prescribed beta-3 agonist is vibegron (Gemtesa). Both medications work through the same biological pathway but offer different choices within the same drug class.
Potential Side Effects and Contraindications
While beta-3 agonists are a targeted therapy, they can still produce side effects. A noted potential side effect is an increase in blood pressure. For this reason, healthcare providers monitor a patient’s blood pressure when starting the medication, especially for individuals who already have hypertension.
Other reported side effects are often mild and can include:
- Headaches
- Urinary tract infections (UTIs)
- Nasopharyngitis (cold-like symptoms)
- Constipation
- Diarrhea
- A rapid heartbeat
Beta-3 agonists should be used with caution or avoided in certain situations. These drugs are contraindicated in patients with severe, uncontrolled high blood pressure. Caution is also advised for individuals with significant liver or kidney disease, as these conditions can affect how the drug is processed. Patients with a clinical bladder outlet obstruction should also be monitored carefully due to a potential for urinary retention.
Distinctions from Anticholinergic Drugs
Beta-3 agonists represent a different approach to treating OAB compared to the older class of medications known as anticholinergics. The primary distinction is their mechanism of action. Anticholinergic drugs (e.g., oxybutynin, tolterodine) work by blocking a neurotransmitter that signals bladder muscles to contract. In contrast, beta-3 agonists actively promote relaxation of the bladder muscle.
This difference in mechanism leads to a different side effect profile. Anticholinergics are known for causing systemic effects like dry mouth, constipation, blurred vision, and drowsiness. A notable concern for older adults is the potential for cognitive side effects, or “brain fog.” Beta-3 agonists do not cause these characteristic anticholinergic side effects, making them a more tolerable option for many patients.
While beta-3 agonists can affect blood pressure, anticholinergics rarely do. The choice between these two classes of medication depends on a patient’s health profile, their tolerance for potential side effects, and their specific OAB symptoms. The availability of beta-3 agonists has provided an alternative for those who do not respond well to or cannot tolerate anticholinergic therapies.