Phentermine is a stimulant used short-term to help with weight loss. Like all medicines, it can cause side effects. Here’s a practical overview:
Common (most people may notice these)
- Dry mouth
- Insomnia or trouble sleeping
- Restlessness, irritability, or feeling anxious
- Dizziness or lightheadedness
- Headache
- Increased heart rate (tachycardia) and/or increased blood pressure
- Constipation or upset stomach
- Sweating
Less common but notable
- Changes in taste
- Frequent urination or urinary problems
- Tremor
- Nervousness or agitation
Serious side effects (seek medical help right away)
- Chest pain, severe shortness of breath, fainting, or a very fast/irregular heartbeat
- Severe or worsening headache, confusion, or dizziness with fainting
- Signs of very high blood pressure (severe headache, blurred vision)
- Signs of a stroke or blood circulation problems (sudden numbness/weakness on one side, trouble speaking, trouble seeing)
- Severe allergic reaction (rash, swelling, trouble breathing)
- Seizures or a significant change in mood, behavior, or thoughts
- Symptoms of panic or agitation that are uncontrollable
Rare but important
- Serotonin syndrome (especially if combined with certain antidepressants or other serotonergic drugs) — symptoms include agitation, confusion, rapid heart rate, high blood pressure, dilated pupils, muscle rigidity, or diarrhea/fever
- Worsening of preexisting conditions like glaucoma or thyroid problems (if you have these, discuss with your clinician)
Who should not take phentermine (contraindications)
- Pregnancy or breastfeeding
- Uncontrolled high blood pressure, severe coronary disease, heart rhythm problems, or congestive heart failure
- Hyperthyroidism
- Glaucoma
- History of drug abuse or being in a position where misuse could be harmful
- Taking a monoamine oxidase inhibitor (MAOI) or have used an MAOI in the last 14 days
- Severe anxiety, agitation, or other major psychiatric disorders (unless supervised by a clinician)
Important cautions and tips
- Phentermine is typically prescribed for short-term use (often up to 12 weeks) because of risk of abuse and diminishing effectiveness over time.
- It can raise blood pressure and heart rate; EMS/monitoring may be needed if you have cardiovascular risk.
- Avoid or limit other stimulants (caffeine, other weight-loss or ADHD meds) while taking phentermine, as they can worsen side effects.
- Take it as directed by your clinician, usually once daily in the morning to avoid insomnia. Stay hydrated and consider a high-fiber diet to help with digestion.
- If you miss a dose, know your doctor’s guidance—some regimens don’t require a missed-dose adjustment, others do.
- If you have existing medical conditions or take other medications, discuss interactions with your prescriber.
What to do if you’re concerned about side effects
- Start with talking to your prescriber or pharmacist. They can assess whether the side effects are tolerable or if dosing adjustments are needed.
- Seek urgent care if you develop chest pain, trouble breathing, severe headache, vision changes, fainting, seizures, signs of an allergic reaction, or any other alarming symptoms.
If you’d like, tell me your country (brand names vary a bit) and any conditions or medications you’re taking, and I can tailor this to your situation and safety considerations. This is general information and not a substitute for medical advice.