Phentermine - Clinical Pharmacology
Phentermine hydrochloride is a sympathomimetic amine with pharmacologic activity similar to the prototype drugs of this class used in obesity, the amphetamines. Actions include central nervous system stimulation and elevation of blood pressure. Tachyphylaxis and tolerance have been demonstrated with all drugs of this class in which these phenomena have been looked for.
Drugs of this class used in obesity are commonly known as "anorectics" or "anorexigenics". It has not been established that the action of such drugs in treating obesity is primarily one of appetite suppression. Other central nervous system actions, or metabolic effects may be involved, for example.
Adult obese subjects instructed in dietary management and treated with "anorectic" drugs, lose more weight on the average than those treated with placebo and diet, as determined in relatively short-term clinical trials.
The magnitude of increased weight loss of drug-treated patients over placebo-treated patients is only a fraction of a pound a week. The rate of weight loss is greatest in the first weeks of therapy for both drug and placebo subjects and tends to decrease in succeeding weeks. The possible origins of the increased weight loss due to the various drug effects are not established. The amount of weight loss associated with the use of an "anorectic" drug varies from trial to trial, and the increased weight loss appears to be related in part to variables other than the drugs prescribed, such as the physician-investigator, the population treated, and the diet prescribed. Studies do not permit conclusions as to the relative importance of the drug and non-drug factors on weight loss.
The natural history of obesity is measured in years, whereas the studies cited are restricted to a few weeks’ duration; thus, the total impact of drug-induced weight loss over that of diet alone must be considered clinically limited.
Side Effects
Along with its needed effects, Phentermine may cause unwanted side effects, some being symptoms of serious heart or lung problems.
Please consult your doctor immediately if you notice:
- Chest pain
- Decreased ability to exercise
- Fainting
- Swelling of feet or lower legs
- Trouble in breathing
Other side effects may occur that usually do not need medical attention. These side effects may go away during treatment as your body adjusts to the medicine.
Other Precautions:
1. If you have heart disease, high blood pressure, arteriosclerosis, hyperthyroidism, diabetes, glaucoma, or a history of drug abuse, tell your doctor before taking phentermine.
2. Tell your doctor what prescription and nonprescription medications you are taking, especially guanethidine, insulin, and MAO inhibitors, even if you stopped taking them within the past two weeks.
3. If you have diabetes, you may need a larger dose of insulin while taking phentermine. Consult your doctor if you have questions or problems.
4. Women who are pregnant should inform their doctors before taking phentermine. If you become pregnant while taking it, contact your doctor promptly.
5. Do not allow anyone else to take this medication.
What should I avoid while taking phentermine?
Avoid drinking alcohol. It can increase some of the side effects of phentermine. Phentermine can cause side effects that may impair your thinking or reactions. Be careful if you drive or do anything that requires you to be awake and alert.
Missed Dose
If you miss a dose, do not double the next dose. Instead, skip the missed dose and resume your usual dosing schedule.
Phentermine Dosage and Administration
Dosage should be individualized to obtain an adequate response with the lowest effective dose.
Exogenous Obesity:
CAPSULES: The usual dosage is 15 mg to 30 mg at approximately 2 hours after breakfast for appetite control. Late evening medication should be avoided because of the possibility of resulting insomnia.
Administration of one capsule (30 mg) daily has been found to be adequate in depression of the appetite for 12 to 14 hours.
TABLETS: The usual adult dose is one tablet (37.5 mg) daily, administered before breakfast or 1 to 2 hours after breakfast. The dosage may be adjusted to the patient's need. For some patients ½ tablet (18.75 mg) daily may be adequate, while in some cases it may be desirable to give ½ tablet (18.75 mg) two times a day.
Phentermine hydrochloride is not recommended for use in patients 16 years of age and under.
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