phentermine hydrochloride
Adipex-P, Ionamin, Pro-Fast HS, Pro-Fast SA, Pro-Fast SR

Available forms
Available by prescription only
Capsules: 18.75 mg, 30 mg, 37.5 mg
Tablets: 8 mg, 37.5 mg
Capsules (resin complex, sustained-release): 15 mg, 30 mg

Indications and dosages
 Short-term adjunct in exogenous obesity. Adults: 8 mg P.O. t.i.d. 30 minutes before meals. Or, 15 to 37.5 mg daily before breakfast or 10 to 14 hours before bedtime. For Adipex-P, 18.75 to 37.5 mg daily before breakfast or 1 to 2 hours after breakfast. For Pro-Fast HS and Pro-Fast SR, 18.75 to 37.5 mg daily 2 hours after breakfast.

Pharmacodynamics
Anorexigenic action: Indirect-acting sympathomimetic amine; causes fewer and less severe adverse reactions from CNS stimulation than do amphetamines, and potential for addiction is lower. Anorexigenic effects are thought to follow direct stimulation of the hypothalamus; they may involve other CNS and metabolic effects.

Pharmacokinetics
Absorption: Absorbed readily after oral administration.
Distribution: Distributed widely throughout body.
Metabolism: No information available.
Excretion: Excreted in urine.

Contraindications and precautions
Contraindicated in patients hypersensitive to sympathomimetic amines, patients with idiosyncratic reactions to them, agitated patients, and patients with hyperthyroidism, moderate to severe hypertension, advanced arteriosclerosis, symptomatic CV disease, or glaucoma. Also contraindicated within 14 days of MAO inhibitor therapy. Use cautiously in patients with mild hypertension. Don’t use in patients with existing heart valve abnormalities or heart murmur.

Interactions
Drug-drug. Acetazolamide, antacids, sodium bicarbonate: Increases renal reabsorption of phentermine and prolongs duration of action. Monitor patient closely.
General anesthetics: Causes arrhythmias. Monitor patient closely.
Guanethidine, other antihypertensives: Decreases hypotensive effects. Monitor blood pressure.
Haloperidol, phenothiazines: Decreases phentermine effects. Monitor patient for clinical effect.
Insulin: Alters insulin requirements in diabetic patient. Monitor blood glucose levels.
MAO inhibitors (drugs with MAO-inhibiting effects), SSRIs: Concomitant use or use within 14 days may cause hypertensive crisis. Avoid use together.
Drug-food. Caffeine: Causes additive CNS stimulation. Discourage excessive use.
Drug-lifestyle. Alcohol use: Increases risk of CNS adverse events. Discourage alcohol use.

Adverse reactions
CNS: overstimulation, headache, euphoria, dysphoria, dizziness, insomnia.
CV: palpitations, tachycardia, increased blood pressure.
GI: dry mouth, dysgeusia, constipation, diarrhea, other GI disturbances.
GU: impotence, dysuria, polyuria, urinary frequency.
Skin: urticaria.
Other: altered libido.

Effects on lab test results
None reported.

Overdose and treatment
Toxicity may cause restlessness, tremor, hyperreflexia, fever, tachypnea, dizziness, confusion, aggressive behavior, hallucinations, blood pressure changes, arrhythmias, nausea, vomiting, diarrhea, and cramps. Fatigue and depression usually follow CNS stimulation; then possible seizures, coma, and death.
 Chlorpromazine may antagonize CNS stimulation. Acidification of urine may hasten excretion.

Special considerations
• Intermittent courses of treatment (6 weeks on, 4 weeks off) are as effective as continuous use.
• Greatest weight loss occurs in first weeks of therapy and loss diminishes in succeeding weeks. When such tolerance develops, drug should be discontinued instead of increasing dosage.
• Don’t crush sustained-release forms.
Pediatric patients
• Drug isn’t recommended for children younger than age 16.

Patient education
• Advise patient not to crush or chew sustained-release products, and to avoid caffeine.
• Tell patient to take last daily dose at least 10 hours before bedtime to prevent insomnia.
• Warn patient not to take drug more frequently than prescribed.
• Advise patient that drug may produce dizziness, fatigue, or drowsiness.
• Tell patient to call if palpitations occur.
• Tell diabetic patients to closely monitor blood glucose level and that adjustment in eating habits, body weight, and activity and change in dosage of antidiabetic may be needed.

Reactions may be common, uncommon, life-threatening, or COMMON AND LIFE THREATENING.
◆ Canada only
◇ Unlabeled clinical use