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All about: Phentermine HCl

Big Image Pronouncation: (FEN-ter-meen)
Class: Anorexiant

Trade Names:
Phentermine HCl
- Tablets 8 mg (equiv. to 6.4 mg base)
- Tablets 30 mg (equiv. to 24 mg base)
- Tablets 37.5 mg (equiv. to 30 mg base)
- Capsules 15 mg (equiv. to 12 mg base)
- Capsules 18.75 mg (equiv. to 15 mg base)
- Capsules 37.5 mg (equiv. to 30 mg base)

Trade Names:
- Capsules 15 mg phentermine resin
- Capsules 15 mg (as resin complex)
- Capsules 30 mg (as resin complex)

Trade Names:
- Tablets 37.5 mg (equiv. to 30 mg base)
- Capsules 37.5 mg (equiv. to 30 mg base)

Trade Names:
Obe-Nix 30
- Capsules 37.5 mg (equiv. to 30 mg base)

Trade Names:
Phentermine Resin
- Capsules 15 mg (as resin complex)
- Capsules 30 mg (as resin complex)

Mechanism of Action


May stimulate satiety center in brain, causing appetite suppression.



Phentermine HCl metabolism is hepatic; it is not significantly biotransformed.


Phentermine HCl excretion is primarily renal (70% to 80% excreted unchanged); excretion is increased by acidifying the urine. The t 1/ 2 is 19 to 24 hr.

Indications and Usage

Short-term (few weeks) adjunct to diet plan to reduce weight.


Hypersensitivity to sympathomimetic amines; advanced arteriosclerosis; symptomatic cardiovascular disease; moderate to severe hypertension; hyperthyroidism; glaucoma; agitated states; history of drug abuse; during or within 14 days following the administration of an MAOI.

Dosage and Administration

8 Mg Dose
Adults and children (greater than or equal to 12 yr)

PO 8 mg up to 3 times daily ½ hr before meals.

15 to 37.5 Mg Dose
Adults and children (greater than 16 yr)

PO 15 to 37.5 mg as a single dose before breakfast or 2 hr after breakfast.

General Advice

  • Discontinue MAO inhibitors at least 14 days before initiating therapy.


Store at controlled room temperature (59° to 86°F).

Drug Interactions


May decrease hypotensive effect of guanethidine.

MAOIs (eg, phenelzine); furazolidone

May cause hypertensive crisis and intracranial hemorrhage.

Selective serotonin reuptake inhibitors (eg, fluoxetine)

Sympathomimetic effects of phentermine and risk of “serotonin syndrome” may be increased.

Laboratory Test Interactions

None well documented.

Adverse Reactions


Regurgitant cardiac valvular disease; palpitation; tachycardia; hypertension.


Overstimulation; restlessness; dizziness; insomnia; euphoria; dysphoria; tremor; headache; psychotic episodes.


Allergic urticaria.


Dry mouth; unpleasant taste; diarrhea; constipation.


Impotence; changes in libido.


Primary pulmonary hypertension.



Category C .




8 mg dose

Safety and efficacy not established in children less than 12 yr.

16 mg dose

Safety and efficacy not established in children 16 yr or less.

Special Risk Patients

Use with caution in patients with glaucoma, hypertension, and diabetes mellitus.

Drug dependence

Psychological and physical dependence may occur with continued use; this class of drugs has been extensively abused.


Tolerance to the anorectic effect usually develops within a few weeks.

Primary pulmonary hypertension

Has been reported with concurrent use of phentermine and fenfluramine or dexfenfluramine.

Valvular heart disease

Serious regurgitant cardiac valvular disease has been reported with concurrent use of phentermine and fenfluramine or dexfenfluramine.



Restlessness, tremor, hyperreflexia, rapid respiration, confusion, assaultiveness, hallucinations, panic states, fatigue, depression, arrhythmias, hypertension, hypotension, circulatory collapse, nausea, vomiting, diarrhea, abdominal cramps, convulsions, coma, death.

Patient Information

  • Explain name, dose, action, and potential side effects of drug.
  • Advise patient using 8 mg dose to take up to 3 times daily as prescribed, ½ hour before meals, and to take last dose several hours before bedtime.
  • Advise patient using 15 to 37.5 mg dose as a single daily dose before, or 2 hr after, breakfast. Advise patient to swallow tablet or capsule whole and to not crush or chew the tablet or capsule.
  • Encourage patient to follow medically supervised weight reduction program. Emphasize that this medication will only work in conjunction with a caloric-restricted diet and exercise program.
  • Advise patient that medication should be taken as prescribed and to not stop taking or change the dose unless advised to do so by health care provider.
  • Explain that appetite suppressant effects are temporary and tolerance to medication and dependence can occur. Caution patient to not increase the dose in an effort to overcome the tolerance when it occurs.
  • Remind patients with diabetes to monitor blood sugar more frequently while implementing dietary restrictions and to notify health care provider if significant changes in blood sugar occur.
  • Caution patient that drug may impair the ability to drive or perform other tasks requiring mental alertness.
  • Advise patient to notify health care provider immediately if the following symptoms occur: decreased exercise tolerance, dyspnea, swelling of the feet or ankles, fainting, chest pain, palpitations, nervousness, or dizziness.
  • Advise patient to notify health care provider if experiencing any unusual or unexplained symptoms.
  • Advise women to inform health care provider if they become pregnant, plan on becoming pregnant, or are breastfeeding.
  • Warn patient not to take any prescription or OTC drugs or dietary supplements without consulting health care provider.
  • Advise patient that follow-up visits may be necessary to monitor therapy and to be sure to keep appointments.

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