From the Guidelines
Phentermine (Adipex) is associated with several common side effects, including dry mouth, insomnia, dizziness, irritability, and elevated heart rate.
Common Side Effects
- Dry mouth
- Insomnia
- Dizziness
- Irritability
- Elevated heart rate These side effects are consistently reported across multiple studies, including those published in Diabetes Care in 2024 1, 2023 1, 2022 1, 2019 1, and 2018 1.
Possible Safety Concerns
Phentermine is contraindicated for use in combination with monoamine oxidase inhibitors, and there is a risk of severe hypertension 1.
Dosage and Administration
The typical adult maintenance dose of phentermine is 8–37.5 mg q.d., with an average wholesale price of $5–$56 for a 30-day supply 1. It is essential to carefully evaluate the potential benefits and risks of phentermine therapy, particularly in patients with pre-existing medical conditions or those taking concomitant medications.
From the FDA Drug Label
The following adverse reactions to phentermine have been identified: Cardiovascular: Primary pulmonary hypertension and/or regurgitant cardiac valvular disease, palpitation, tachycardia, elevation of blood pressure, ischemic events. Central Nervous System: Overstimulation, restlessness, dizziness, insomnia, euphoria, dysphoria, tremor, headache, psychosis Gastrointestinal: Dryness of the mouth, unpleasant taste, diarrhea, constipation, other gastrointestinal disturbances. Allergic: Urticaria. Endocrine: Impotence, changes in libido.
The side effects of Phentermine (Adipex) include:
- Cardiovascular side effects such as primary pulmonary hypertension, palpitation, and elevation of blood pressure
- Central Nervous System side effects such as overstimulation, restlessness, and insomnia
- Gastrointestinal side effects such as dryness of the mouth and diarrhea
- Allergic side effects such as urticaria
- Endocrine side effects such as impotence and changes in libido 2, 2, 2
From the Research
Side Effects of Phentermine (Adipex)
The side effects of Phentermine (Adipex) can be significant and varied. Some of the reported side effects include:
- Severe headaches 3
- Hypertension 3
- Depressive symptoms 3
- Breathlessness 3
- Palpitations with irritability 3
- Atrial fibrillation (AF) 4
- Psychomotor side effects, such as those induced by phentermine alone, which can be partially reversed by combining phentermine with 5-HTP/Carbidopa 5
Cardiovascular Side Effects
Long-term use of phentermine may be associated with severe cardiovascular side effects, including:
- Increased risk of cardiovascular events, although large-scale observational studies have not identified an increased risk 6
- Elevations in blood pressure, although recent research has not found a significant association 6
- Atrial fibrillation, a rare but potentially serious side effect 4
Drug-Drug Interactions
Phentermine can interact with other medications, leading to adverse drug reactions (ADRs) and drug-drug interactions (DDIs). Some of the potential interactions include:
- Moderate and severe DDIs with co-prescribed drugs, such as dextromethorphan and tramadol 7
- Potential interactions with other pharmaceutical constituents, which may increase the risk of ADRs and DDIs 7
Weight Loss and Efficacy
Phentermine can lead to significant weight loss, particularly when used in combination with other medications, such as 5-HTP/Carbidopa 5. However, the individual response to phentermine is unpredictable and may be influenced by factors other than plasma drug concentration 3.