Quick answer: The most common phentermine side effects are dry mouth, trouble sleeping, restlessness, a faster heartbeat, headache and constipation. They come from the way phentermine works: it is a stimulant related to amphetamine, which is also why it raises blood pressure and heart rate and why it is a Schedule IV controlled substance. The FDA label lists these effects without saying how often each one happens, so be wary of any page that quotes precise percentages for phentermine alone.
Key takeaways
- Common: dry mouth, insomnia, restlessness, palpitations or a racing heart, headache, dizziness, constipation or diarrhea, an unpleasant taste.
- Serious but rare: primary pulmonary hypertension and heart-valve disease, reported mainly with the old fen-phen combination. The label says a link with phentermine alone can't be ruled out.
- Stop and call right away for: chest pain, shortness of breath that's new or getting worse, fainting, or swelling in the legs or ankles.
- Not for: heart disease, uncontrolled high blood pressure, an overactive thyroid, glaucoma, a history of drug misuse, pregnancy or breastfeeding, or within 14 days of an MAOI.
The most common side effects
| Side effect | What helps | When to call |
|---|---|---|
| Dry mouth, unpleasant taste | Water through the day, sugar-free gum | If it stops you eating or drinking normally |
| Trouble sleeping | Take the dose in the morning, not later in the day; no caffeine after noon | If it lasts past the first couple of weeks |
| Restlessness, jitteriness | Cut back on caffeine | If you feel agitated, or your mood changes |
| Racing heart, palpitations | Stop caffeine and pre-workout stimulants | Straight away if it comes with chest pain, shortness of breath or fainting |
| Headache, dizziness | Fluids and regular meals | If it's severe, or comes with a high blood pressure reading |
| Constipation or diarrhea | Fiber, fluids, a daily walk | If it lasts more than a few days |
Source: the phentermine prescribing information (adverse reactions). The "What helps" and "When to call" columns are the clinic's guidance.
Serious side effects: stop and call
- Primary pulmonary hypertension, a rare and often fatal lung disease. It was reported mainly when phentermine was combined with fenfluramine or dexfenfluramine (fen-phen). The label says a link with phentermine alone can't be ruled out. Early signs are new or worsening shortness of breath, chest pain, fainting and swelling in the legs.
- Heart-valve disease, also reported mainly with fen-phen.
- Rising blood pressure and heart rate. The label asks for caution even with mild high blood pressure. We check both before you start and when you come in to the clinic.
- Tolerance. If the appetite effect fades, the label says not to raise the dose; the medication should be stopped.
- Mood and thinking changes. The label lists euphoria, dysphoria and, rarely, psychosis.
Phentermine side effects in women
Phentermine is contraindicated in pregnancy and while breastfeeding, so use reliable contraception while taking it and stop if you become pregnant. The label also lists changes in libido. Beyond that, the side effects are the same for women and men.
Does phentermine cause hair loss? Constipation?
Hair loss isn't on the phentermine label. When people notice shedding while losing weight on any program, the usual cause is telogen effluvium, which follows rapid weight loss or eating too little, not the medication itself. Constipation is on the label; fiber, fluids and movement usually handle it.
How long do the side effects last?
Dry mouth and trouble sleeping are usually worst in the first days to weeks. For the timeline, see how long it takes phentermine to work, which covers the first weeks in detail.
Who should not take phentermine
From the label: a history of heart disease (coronary artery disease, stroke, arrhythmias, heart failure, uncontrolled high blood pressure); within 14 days of a monoamine oxidase inhibitor (MAOI); an overactive thyroid; glaucoma; agitated states; a history of drug abuse; pregnancy; breastfeeding; a known reaction to stimulant medications.
Interactions and alcohol
- MAOIs: never within 14 days, because of the risk of a hypertensive crisis.
- Alcohol: the label warns that combining them may cause an adverse reaction. Avoid it.
- Diabetes medicines: insulin or oral diabetes medicines may need lowering as you lose weight.
- Some blood pressure medicines: phentermine can weaken adrenergic neuron blockers.
- Other weight-loss medicines: the label does not recommend combining phentermine with them, because the combinations have not been studied for safety.
Is phentermine safer than Ozempic?
They carry different risks: phentermine's are cardiovascular and stimulant-related, while GLP-1 medications' are mainly digestive, with a boxed warning about thyroid C-cell tumors. Which is safer depends on your heart, blood pressure and history. See phentermine vs GLP-1.
How phentermine is monitored in California
Phentermine is a Schedule IV controlled substance. California law requires the prescriber to check the state's prescription-monitoring database (CURES) before the first prescription and at least every six months while it continues (Health and Safety Code section 11165.4). We also check blood pressure and heart rate before you start and when you come in to the clinic.
Frequently asked questions
What are the dangers of taking phentermine?
The serious risks are rare: primary pulmonary hypertension and heart-valve disease, reported mainly with the old fen-phen combination, and rises in blood pressure and heart rate. Stop and call right away for chest pain, new or worsening shortness of breath, fainting, or swelling in the legs or ankles.
Can I drink alcohol on phentermine?
It is best not to. The label warns that combining alcohol with phentermine may cause an adverse reaction.
Why won't some doctors prescribe phentermine?
It is a stimulant and a Schedule IV controlled substance, it raises blood pressure and heart rate, and the label approves it for short-term use only. Some clinicians prefer other options, especially for people with heart disease or high blood pressure.
What happens if the appetite effect wears off?
That is tolerance. The label says not to raise the dose to chase the effect; the medication should be stopped instead. Talk to your clinician about what comes next.
Will phentermine affect my mood?
It can. The label lists restlessness, euphoria and dysphoria, and rarely psychosis. Call your clinician about any mood change.
More on phentermine
How the phentermine program at our clinic works, phentermine dosing and timing, whether phentermine burns fat, and how progress is tracked with an InBody scan.
Related treatments at our Mission Viejo clinic
Not sure which treatment fits? These are the related services our clinicians most often discuss at a consultation.
- Appetite Suppressants Orange County, We offer prescription appetite suppressants for weight loss in Orange County.
- B12 & Lipotropic Fat-Burning Shots, Try B12 Fat Burning Injections for Weight Loss journey.
- Medical Weight Loss Orange County, Clinician-supervised medical weight loss in Mission Viejo, CA.
References
- Phentermine hydrochloride prescribing information (Lomaira): contraindications, warnings, adverse reactions, drug interactions — FDA label via DailyMed (revised 2026)
- Phentermine — MedlinePlus, U.S. National Library of Medicine
- Pharmacological management of obesity: an endocrine Society clinical practice guideline — J Clin Endocrinol Metab 2015, via PubMed
- California Health and Safety Code section 11165.4 (CURES consultation) — California Legislative Information