Phentermine's side effects are stimulant effects — heart rate, blood pressure, sleep and mood — and its label lists them without a single percentage. That is not an omission by this page. Phentermine's label is an older-format label without a clinical-trial adverse-reaction table, and it describes reactions by body system only. What it does carry are two serious warnings rooted in the fen-phen episode of the 1990s, and a dependence warning, because the drug is related to amphetamine.
This page reads the US label for generic phentermine 37.5 mg (Aurolife, effective 10 June 2026) and the Lomaira 8 mg label (effective 21 May 2026), read 3 November 2026, with the published studies and FDA's adverse event reports. The drug's overall record, trials and cost are on phentermine.
What the label lists
The label's adverse-reactions section, in full by category:
| Body system | Reactions listed |
|---|---|
| Cardiovascular | Primary pulmonary hypertension and/or regurgitant heart-valve disease, palpitations, fast heart rate (tachycardia), raised blood pressure, ischaemic events |
| Central nervous system | Overstimulation, restlessness, dizziness, insomnia, euphoria, dysphoria, tremor, headache, psychosis |
| Gastrointestinal | Dry mouth, unpleasant taste, diarrhoea, constipation, other gastrointestinal disturbances |
| Allergic | Urticaria (hives) |
| Endocrine | Impotence, changes in libido |
No frequency is given for any of them. The label does not say how many people in a trial had insomnia or dry mouth, and it has no placebo comparison. That makes phentermine unusual among weight-loss drugs: Qsymia, Contrave and orlistat labels all carry incidence tables, as do the GLP-1 labels.
The nearest thing to rates for phentermine comes from the combination drug. Qsymia's label table shows dry mouth in 19%, insomnia in 9% and dizziness in 9% on its top strength (15 mg phentermine with 92 mg topiramate), against 3%, 5% and 3% on placebo. Those figures mix two drugs and cannot be assigned to phentermine alone.
The two serious warnings
Primary pulmonary hypertension (PPH) — described in the label as "a rare, frequently fatal disease of the lungs". It was reported in people using phentermine with fenfluramine or dexfenfluramine; the label says a link with phentermine alone "cannot be ruled out", noting rare cases in people who reportedly used phentermine alone. The label names the first symptom as usually breathlessness, with chest pain, fainting or leg swelling also possible, and says new, unexplained symptoms of this kind call for stopping the drug and an evaluation.
Regurgitant heart-valve disease — mainly affecting the mitral, aortic and tricuspid valves, reported in otherwise healthy people who used phentermine with fenfluramine or dexfenfluramine. The label says phentermine's role "has not been established" and that the course after stopping is unknown, but that a link with phentermine alone "cannot be ruled out".
Where those warnings came from
In August 1997, Mayo Clinic cardiologists described 24 women (mean age 44) who developed unusual valve disease about a year after starting fenfluramine–phentermine; all had abnormal valves and leakage on echocardiography, eight had newly documented pulmonary hypertension, and five had needed surgery. The valves looked like those damaged by carcinoid tumours or ergot drugs (Connolly et al., NEJM 1997). The paper notes that US prescriptions for the two drugs exceeded 18 million in 1996, though the combination was never approved.
On 15 September 1997, FDA asked for fenfluramine (Pondimin) and dexfenfluramine (Redux) to be withdrawn, citing valvular heart disease, as recorded in FDA's later Federal Register notice (29 September 2015). Phentermine stayed on the market, and the valve and lung warnings stayed on its label.
What later data show
The largest study of longer use followed 13,972 adults who started phentermine in 2010–2015 in US health systems. Heart disease or death within up to three years was rare — 0.3%, 41 events — with no significant difference between people who used it for three months or less and those who used it longer (Lewis et al., Obesity 2019). The cohort was 84% women with a mean age of 43.5, and people with heart disease are usually not prescribed phentermine, so it describes a low-risk group. It is observational and cannot exclude a small risk.
Blood pressure, heart rate and who should not use it
Raised blood pressure, palpitations and a fast heart rate are on the label's list, and the "typical actions of amphetamines" it describes include raised blood pressure. That is why the label contraindicates phentermine in people with a history of cardiovascular disease — coronary artery disease, stroke, arrhythmias, heart failure or uncontrolled hypertension — as well as hyperthyroidism, glaucoma, agitated states, a history of drug abuse, pregnancy, nursing, and use of MAO inhibitors within 14 days (the combination risks a hypertensive crisis). How GLP-1 medicines affect blood pressure, mostly lowering it, is on GLP-1 and blood pressure.
Sleep, mood and the nervous system
Insomnia is the one effect the label builds into its dosing notes: it says late-evening use should be avoided. Restlessness, overstimulation, tremor, euphoria or low mood (dysphoria), and, rarely, psychosis are on the list. The label also warns that phentermine may impair driving or operating machinery. Of nine current GLP-1 labels, one lists insomnia; that record is on GLP-1 insomnia.
Dependence, tolerance and stopping
- Schedule IV controlled substance. The label: phentermine is related chemically and pharmacologically to the amphetamines, which "have been extensively abused".
- Tolerance. The label says tolerance to the appetite effect "usually develops within a few weeks", and that when it does, the drug should be stopped rather than the dose increased.
- Stopping after heavy use. Abrupt cessation after prolonged high-dose use "results in extreme fatigue and mental depression", with sleep changes.
- Chronic intoxication. The label lists severe skin conditions, marked insomnia, irritability, hyperactivity and personality changes, and in its most severe form a psychosis "often clinically indistinguishable from schizophrenia".
Overdose
The label lists restlessness, tremor, overactive reflexes, rapid breathing, confusion, aggression, hallucinations and panic, followed by fatigue and depression; heart-rhythm problems, high or low blood pressure and circulatory collapse; and nausea, vomiting, diarrhoea and cramps. It says overdose of similar drugs has been fatal. Suspected overdose is a medical emergency. What the GLP-1 labels say about overdose is on GLP-1 overdose.
What FDA's report database holds
FDA's Adverse Event Reporting System, searched through openFDA on 3 November 2026 (data to 28 October 2026), holds 6,506 reports that list phentermine hydrochloride among the drugs a person took; 3,668 are classed as serious. The most frequent terms:
| Term in the report | Reports |
|---|---|
| Drug ineffective | 565 |
| Nausea | 531 |
| Pain | 482 |
| Fatigue | 365 |
| Headache | 347 |
| Anxiety | 330 |
| Vomiting | 276 |
| Dizziness | 235 |
| Insomnia | 172 |
| Mitral valve incompetence (leakage) | 25 |
| Pulmonary hypertension | 22 |
| Pulmonary arterial hypertension | 4 |
How to read this: a report lists every drug a person was using, so many of these involve other medicines, and anyone can file one. Report counts cannot show that phentermine caused an event, or how often it happens — they show what has been reported. The search excluded the phentermine/topiramate combination. Nausea and vomiting ranking high, for example, does not match the label's list and may reflect other drugs in the same reports.
What the record does not show
- Any incidence rate for phentermine's side effects from a placebo-controlled trial on its label.
- Whether phentermine alone causes valve disease or pulmonary hypertension, and at what rate. The label says it cannot be ruled out; the evidence is rare case reports.
- Long-term safety beyond about three years, or safety combined with a GLP-1 medicine. The label advises against combining phentermine with other weight-loss drugs.
The side-effect record for the GLP-1 class, with rates from large trials, is on GLP-1 side effects. Any symptom that worries a person on phentermine — especially breathlessness, chest pain, fainting or swelling — is a matter for their prescriber, urgently where the label says so.
Sources
- Phentermine hydrochloride tablets USP 37.5 mg, US prescribing information, Aurolife Pharma, effective 10 June 2026, set ID 5927e0f6-3de3-4885-a633-5b92003f6fe4 (openFDA drug label record), read 3 November 2026 — sections 2, 4, 5, 6, 9, 10.
- LOMAIRA (phentermine hydrochloride) 8 mg, US prescribing information, effective 21 May 2026, set ID 0718f0f3-77f2-45d7-be12-1ed718c8b806 (openFDA), read 3 November 2026 — Warnings, Precautions, Adverse Reactions, Overdosage.
- QSYMIA prescribing information, Vivus, effective 22 April 2026, set ID 40dd5602-53da-45ac-bb4b-15789aba40f9 — Table 1.
- Connolly HM et al. "Valvular heart disease associated with fenfluramine-phentermine." NEJM 1997;337(9):581–8, doi:10.1056/NEJM199708283370901, PMID 9271479.
- Federal Register, 29 September 2015, document 2015-24619, on the 1997 withdrawal of PONDIMIN and PONDEREX (govinfo), read 3 November 2026.
- Lewis KH et al. "Safety and Effectiveness of Longer-Term Phentermine Use." Obesity 2019;27(4):591–602, doi:10.1002/oby.22430, PMID 30900410.
- FDA Adverse Event Reporting System via openFDA drug/event endpoint, search
patient.drug.openfda.generic_name.exact:"PHENTERMINE HYDROCHLORIDE", data last updated 28 October 2026, read 3 November 2026.