Skip to Content

Fastin Side Effects

Generic Name: phentermine

Note: This page contains information about the side effects of phentermine. Some of the dosage forms included on this document may not apply to the brand name Fastin.

For the Consumer

Applies to phentermine: oral capsule, oral capsule extended release, oral tablet, oral tablet disintegrating

In addition to its needed effects, some unwanted effects may be caused by phentermine (the active ingredient contained in Fastin). In the event that any of these side effects do occur, they may require medical attention.

Major Side Effects

You should check with your doctor immediately if any of these side effects occur when taking phentermine:

  • Seeing, hearing, or feeling things that are not there
  • severe mental changes
Incidence not known:
  • Chest pain
  • decreased ability to exercise
  • dizziness
  • fainting
  • fast, irregular, pounding, or racing heartbeat or pulse
  • headache
  • numbness or tingling in the arms or legs
  • swelling of the feet or lower legs
  • trembling or shaking of the legs, arms, hands, or feet
  • trouble breathing
  • trouble with thinking, speaking, or walking
  • weakness

Minor Side Effects

Some of the side effects that can occur with phentermine may not need medical attention. As your body adjusts to the medicine during treatment these side effects may go away. Your health care professional may also be able to tell you about ways to reduce or prevent some of these side effects. If any of the following side effects continue, are bothersome or if you have any questions about them, check with your health care professional:

Incidence not known:
  • Decreased interest in sexual intercourse
  • difficulty having a bowel movement (stool)
  • dry mouth
  • false or unusual sense of well-being
  • hives or welts
  • inability to have or keep an erection
  • increased in sexual ability, desire, drive, or performance
  • increased interest in sexual intercourse
  • itching
  • loss in sexual ability, desire, drive, or performance
  • redness of the skin
  • skin rash
  • unpleasant taste

For Healthcare Professionals

Applies to phentermine: compounding powder, oral capsule, oral capsule extended release, oral tablet, oral tablet disintegrating


Cardiovascular adverse effects may be associated with phentermine (the active ingredient contained in Fastin) Palpitations, tachycardia, elevation of blood pressure, ischemic event have been reported. Primary pulmonary hypertension and valvular heart disease has been reported in patients taking phentermine in combination with fenfluramine or dexfenfluramine and in rare cases in patients who have taken phentermine alone. Analysis of available data suggests that fenfluramine and dexfenfluramine are the causal agents, and both drugs have been withdrawn from the market as of September 15, 1997.[Ref]

A 37-year-old woman was admitted to the hospital after an episode of ventricular fibrillation. The patient was being treated for moderate obesity and had been taking phentermine hydrochloride, thyroid (USP), and trichlormethiazide. Her previous medical history had been unremarkable. It was suspected that one or more of her medications contributed to the nearly fatal arrhythmia.

Two case reports describe patients using phentermine (one in combination with phendimetrazine) who later developed ischemic cerebrovascular disease. One suffered a cerebral infarct with angiographic evidence of vasculopathy involving multiple vascular beds. The other patient developed headache and a hemisensory disturbance of seven days duration.

A case of fatal pulmonary hypertension has also been reported in a 29-year-old woman 8 months following the use of fenfluramine (of which dexfenfluramine is the dextro isomer) and phentermine for only 23 days. Histopathological findings upon autopsy indicated severe pulmonary hypertension. She did not smoke and had no family history of pulmonary hypertension. No obvious risk factors other than obesity were present, although preexisting disease was not ruled out.

A case of restrictive cardiomyopathy due to endocardial fibrosis after short term use of fenteramine in combination with phentermine has been reported.[Ref]

Nervous system

Phentermine (the active ingredient contained in Fastin) produces nervous system stimulation, which may result in tremor, anxiety, restlessness, dizziness, insomnia, euphoria, dysphoria, and headache.[Ref]

Fifty women with refractory obesity received phentermine resinate for a period of 20 weeks. Three defaulted from the study due to incapacitating headaches without significant change in blood pressure. Two additional patients defaulted due to complaints of irritability.

A double-blind clinical trial of phentermine and fenfluramine involving 81 people with simple obesity reported several CNS complaints from patients taking phentermine resin during the study (twenty six complaints during the first 6 weeks of the study). The number of complaints diminished with time over the course of the study. Only seven complaints of CNS effects were reported at 20 weeks. Complaints reported by patients include sleep difficulties, nervousness, depression, fatigue and increased dreaming.[Ref]


Rarely psychotic episodes have been reported at recommended doses with some sympathomimetic drugs.[Ref]


Gastrointestinal adverse effects may include dryness of the mouth, unpleasant taste, diarrhea, and constipation.[Ref]


An allergic reaction resulting in urticaria may occur.[Ref]


Endocrine effects such as impotence and changes in libido may occur in patients taking phentermine (the active ingredient contained in Fastin) [Ref]


1. "Product Information. Ionamin (phentermine)." Rhone-Poulenc Rorer, Collegeville, PA.

2. Kokkinos J, Levine SR "Possible association of ischemic stroke with phentermine." Stroke 24 (1993): 310-3

3. U.S. Food and Drug Administration, Center for Drug Evaluation and Research "Reports of valvular heart disease in patients receiving concomitant fenfluramine and phentermine. Available from URL:" ([1997 Jul 8]):

4. Connolly HM, Crary JL, McGoon MD, et al. "Valvular heart disease associated with fenfluramine-phentermine." N Engl J Med 337 (1997): 581-8

5. U.S. Food and Drug Administration, Center for Drug Evaluation and Research ""Fen-phen" update. Available from: URL:" ([1997 Aug]):

6. Mark EJ, Patalas ED, Change HT, Evans RJ, Kessler SC "Fatal pulmonary hypertension associated with short-term use of fenfluramine and phentermine." N Engl J Med 337 (1997): 602-6

7. "Product Information. Suprenza (phentermine)." Akrimax Pharmaceuticals, Cranford, NJ.

8. "Product Information. Fastin (phentermine)." SmithKline Beecham, Philadelphia, PA.

9. U.S. Food and Drug Administration, Center for Drug Evaluation and Research "FDA announces withdrawal fenfluramine and dexfenfluramine (Fen-Phen). Available from: URL: URL:" ([1997 Sept]):

10. Graham DJ, Green L "Further cases of valvular heart disease associated with fenfluramine-phentermine." N Engl J Med 337 (1997): 635

11. Gibbs HR "Diet pills and sudden death." N Engl J Med 318 (1988): 1127

12. Douglas A, Douglas JG, Robertson CE, Munro JF "Plasma phentermine levels, weight loss and side-effects." Int J Obes 7 (1983): 591-5

13. Weintraub M, Hasday JD, Mushlin AI, Lockwood DH "A double-blind clinical trial in weight control. Use of fenfluramine and phentermine alone and in combination." Arch Intern Med 144 (1984): 1143-8

Not all side effects for Fastin may be reported. You should always consult a doctor or healthcare professional for medical advice. Side effects can be reported to the FDA here.

Disclaimer: Every effort has been made to ensure that the information provided is accurate, up-to-date and complete, but no guarantee is made to that effect. In addition, the drug information contained herein may be time sensitive and should not be utilized as a reference resource beyond the date hereof. This material does not endorse drugs, diagnose patients, or recommend therapy. This information is a reference resource designed as supplement to, and not a substitute for, the expertise, skill , knowledge, and judgement of healthcare practitioners in patient care. The absence of a warning for a given drug or combination thereof in no way should be construed to indicate safety, effectiveness, or appropriateness for any given patient. does not assume any responsibility for any aspect of healthcare administered with the aid of materials provided. The information contained herein is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. If you have questions about the substances you are taking, check with your doctor, nurse, or pharmacist.