Can weight loss medication, such as phentermine (anorexiant), cause forgetfulness in a patient?

Phentermine and Forgetfulness

Phentermine monotherapy is not associated with forgetfulness or cognitive impairment based on available evidence. However, when combined with topiramate (as phentermine/topiramate ER), cognitive impairment is a recognized adverse effect attributable to the topiramate component.

Phentermine Alone: No Cognitive Effects

• Phentermine works as a noradrenergic agent that suppresses appetite through norepinephrine elevation in the central nervous system 1
• The most common side effects of phentermine monotherapy include dry mouth, insomnia, dizziness, and irritability—not cognitive dysfunction or forgetfulness 1
• Recent safety reviews of phentermine spanning decades of use in millions of patients have not identified memory problems or cognitive impairment as adverse effects 2
• Large-scale observational studies evaluating phentermine's safety profile have not documented forgetfulness as a concern 3

Phentermine/Topiramate Combination: Cognitive Risk Present

If your patient is taking phentermine/topiramate ER (Qsymia), cognitive impairment IS a documented side effect:

• The FDA-approved combination product lists cognitive impairment as a specific safety concern attributable to topiramate 1
• Common side effects of phentermine/topiramate ER include paresthesias, dizziness, dysgeusia, insomnia, constipation, and dry mouth 1
• The American Diabetes Association specifically warns about cognitive impairment with this combination medication 1
• In clinical practice, mood changes and neuropsychiatric side effects (which can include cognitive symptoms) occur in approximately 25.8% of patients on topiramate-containing regimens 4

Clinical Algorithm for Evaluation

If the patient reports forgetfulness while on weight loss medication:

1. Verify the exact medication: Distinguish between phentermine alone versus phentermine/topiramate combination 1

2. If on phentermine/topiramate ER: The forgetfulness is likely medication-related due to topiramate's known cognitive effects 1. Consider:

   • Dose reduction (from 15/92 mg to 7.5/46 mg) 5
   • Switching to phentermine monotherapy 6
   • Switching to alternative weight loss medications without cognitive effects (liraglutide, semaglutide, naltrexone/bupropion) 1

3. If on phentermine alone: Forgetfulness is unlikely to be medication-related 1, 2. Evaluate for:

   • Other medications causing cognitive effects
   • Sleep deprivation (phentermine can cause insomnia, which secondarily affects cognition) 1
   • Unrelated medical conditions affecting memory

Key Pitfall to Avoid

Do not confuse phentermine monotherapy with phentermine/topiramate combination therapy. The cognitive side effects are specific to topiramate, not phentermine 1. Many patients and providers mistakenly attribute all side effects of the combination product to "phentermine" when topiramate is the actual culprit for cognitive symptoms.