See the DrugPatentWatch profile for Phentermine
What side effects can phentermine cause?
Phentermine can cause a range of side effects, from common stimulant-related effects to more serious cardiovascular or mental health symptoms.
Commonly reported side effects include:
- Fast heartbeat or palpitations
- Increased blood pressure
- Jitteriness, restlessness, or feeling “wired”
- Trouble sleeping (insomnia)
- Dry mouth
- Headache
- Dizziness
- Nausea or stomach discomfort
- Constipation or diarrhea
- Sweating
- Tremor
Some people also notice appetite changes and can feel more alert or restless while taking it, which is part of how the medication works but can still feel unpleasant.
What are the serious phentermine side effects that need urgent care?
Seek urgent medical help if you have signs that suggest a serious reaction, such as:
- Chest pain, fainting, or severe shortness of breath
- Very fast, irregular, or pounding heartbeat
- Severe headache with weakness or vision changes
- Swelling of the face, lips, tongue, or throat, or trouble breathing (possible allergic reaction)
- Severe agitation, confusion, hallucinations, or extreme restlessness
- Signs of high blood pressure crisis (severe headache, chest pain, shortness of breath)
If you experience any of these, stop using phentermine and contact emergency services or a clinician right away.
How long do phentermine side effects last?
Many stimulant-type side effects (like insomnia, restlessness, and jitteriness) are more noticeable early in treatment or after dose increases. For many people, these symptoms improve as the body adjusts, but others experience persistent effects, especially insomnia and palpitations. Because phentermine is typically prescribed short-term, side effects are also expected to resolve after stopping—unless complications (like cardiovascular issues) developed.
Your clinician can suggest strategies such as adjusting the timing of your dose to reduce insomnia, but you should not change dosing without medical guidance.
Who is at higher risk for bad phentermine side effects?
Risk can be higher if you have:
- Heart disease, uncontrolled high blood pressure, or history of arrhythmias
- Hyperthyroidism
- Glaucoma
- Significant anxiety or a history of substance misuse
- Certain psychiatric conditions where stimulants worsen symptoms
Also, drug interactions can raise risk. Phentermine should not be combined with many other medications without clinician input, especially drugs that affect serotonin or certain blood pressure/heart-related therapies.
What phentermine side effects overlap with other weight-loss medicines?
Phentermine’s side effects are similar to other stimulant-like weight-loss or appetite-suppressing drugs. Symptoms like insomnia, jitteriness, and palpitations can be seen across this class.
If you’re switching from another medication, side effects may temporarily persist due to overlap with the prior drug’s effects, but the overall pattern should align with how the new medication works.
What should you do if you think phentermine is causing side effects?
- Contact your prescribing clinician promptly if symptoms are bothersome or worsening, especially palpitations, blood pressure changes, or severe mood/behavior changes.
- Don’t increase the dose to “push through” side effects.
- If symptoms are severe (chest pain, fainting, breathing trouble, allergic symptoms), get emergency care.
If you want, tell me your dose, when you take it, and which side effects you’re experiencing (and how soon after starting or taking it they appear). I can help you sort out what’s more likely expected vs. what warrants urgent evaluation.