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What Factors Affect a Sufficient Phentermine Dosage?

By S. Berger
Updated May 17, 2024
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Phentermine is a medication that is part of the amphetamine class of drugs, and is used for short periods of time to promote weight loss. The main factor affecting the proper phentermine dosage for a given patient is age, since the doses used for adult and pediatric weight loss are usually quite different. Other variables may influence the dosage used for a patient, such as whether this medication is administered in capsule or tablet form. Pre-existing medical conditions that affect renal or hepatic function, or the functioning of the kidneys or liver, respectively, may also influence the dose that is prescribed to a patient.

Adults taking this medication for weight loss usually have a phentermine dosage that consists of 8 milligrams (mg) of the compound, taken half an hour before meals, three times every day, if immediate-release capsules are being taken. Extended-release tablets and phentermine resin are also available that require only one dose per day. The phentermine dosage for these formulations usually consist of 15 mg to 37.5 mg taken upon waking. Due to this drug's ability to cause insomnia, extended-release formulations should not be taken more than 10 to 14 hours before going to bed.

Children may also take this medication for weight loss, and they may be given up to 8 mg of the immediate-release capsules half an hour before eating, three times a day. Phentermine dosage for extended-release preparations such as tablets or resins are usually 15 mg or 18.75 mg, taken upon waking up in the morning. Up to 37.5 mg of the extended-release formulation may be taken by children, but dosages this high are more rare among this population, given the risk of increased side effects.

Some medical conditions, particularly those affecting the liver or kidneys, may increase phentermine's effects. The liver and kidneys are responsible for breaking down and excreting many types of drugs, including phentermine. Damage or compromised functioning of either organ system can therefore prolong how long a standard phentermine dose is active in the affected individual.

For this reason, some doctors may advise that if this medication is to be prescribed to individuals with damage to either the kidneys or liver, the phentermine dosage should be reduced. Decreasing the dose of this drug can help reduce side effects from a buildup of this medication in the blood stream. Usually, a dose of 18.75 mg of the extended-release formulation is used, since lowering an immediate-release dose is not possible with the dosage levels that the immediate-release formulation is manufactured in.

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