Can phentermine (phentermine) cause blurry vision?

Phentermine and Blurry Vision

Yes, phentermine can cause blurry vision as a side effect, and patients should be monitored for visual changes while taking this medication. 1

Mechanism and Ocular Effects

• Phentermine is a sympathomimetic amine anorectic that stimulates the central nervous system, which can affect visual function through multiple mechanisms 1, 2
• As a stimulant medication, phentermine can cause mydriasis (pupillary dilation), which may affect visual acuity and potentially promote angle closure in susceptible patients 3
• Visual disturbances including blurred vision are documented adverse effects that should prompt immediate medical attention 1

Clinical Monitoring Recommendations

• Patients should have baseline visual acuity testing before starting phentermine 1
• At each monthly visit, patients should be questioned about visual disturbances including blurred vision or scotomata (blind spots in the visual field) 1
• Patients should be instructed to contact their physician immediately if they experience any change in vision while taking phentermine 1
• The medication should be discontinued immediately and permanently if there are any signs of visual toxicity 1

Risk Factors and Special Considerations

• Patients with pre-existing glaucoma are at higher risk for visual complications and phentermine is contraindicated in these patients 1
• Higher doses of phentermine increase the risk of ocular side effects 1
• Patients with narrow anterior chamber angles may be at increased risk for angle-closure glaucoma when taking medications like phentermine that can cause mydriasis 3, 4

Combination Therapy Concerns

• The combination of phentermine with topiramate (as in Qsymia) carries additional ocular risks, including acute myopia and ocular pain 1, 5
• Phentermine/topiramate combination therapy has been associated with angle-closure glaucoma, which requires immediate medical attention 5
• When using phentermine/topiramate combination, patients should be monitored for potential consequences of acute angle-closure glaucoma 1

Management of Visual Side Effects

• If blurry vision occurs, phentermine should be discontinued 1
• Ophthalmologic evaluation is recommended for patients experiencing visual changes while on phentermine 4
• Visual side effects are generally reversible upon discontinuation of the medication, but permanent damage can occur if the medication is continued despite symptoms 3

Other Common Side Effects of Phentermine

• Besides visual disturbances, common side effects include dry mouth, insomnia, dizziness, irritability, elevated heart rate, and increased blood pressure 1
• Phentermine is only approved for short-term use (≤12 weeks) due to concerns about cardiovascular effects and potential for dependence 1, 6

Drug Interactions Affecting Vision

• Concurrent use of phentermine with other medications that affect the central nervous system may increase the risk of visual disturbances 6
• Monoamine oxidase inhibitors are contraindicated with phentermine and can increase the risk of adverse effects, including visual disturbances 1

Remember that phentermine is a controlled substance (Schedule IV) with potential for abuse and should be used cautiously and only for short-term treatment of obesity 1.