Phentermine dosage guide: Finding the right dose for weight loss

Weight-loss medications often make headlines these days, especially GLP-1s like Zepbound^® and Wegovy^®. But they’re not the only prescription options available. Phentermine has been part of the conversation for decades, offering an affordable and effective way to help people lose weight.

Phentermine works differently from GLP-1s. Instead of slowing digestion and affecting hunger hormones, it’s a stimulant that helps lower appetite. Approved by the FDA in 1959, it remains one of the most prescribed medications for people looking for help in reducing appetite. It’s available in widely used generic versions as well as in the brand Lomaira, which comes in a lower dose for more flexible use.

While GLP-1 medications have gotta a lot more attention recently, phentermine continues to be popular thanks to its long history, lower cost, and effectiveness for many people. While phentermine can be effective, it’s only FDA-approved for up to 12 weeks; any prescribed use after that is considered off-label. Qsymia^®, a medication that combines phentermine with topiramate, is approved for longer use. 

Phentermine has also stayed popular not just for its long history and affordability but also for its straightforward dosing. Instead of starting low and slowly increasing like many GLP-1 medications, phentermine is usually started at a set dose—typically 15 mg to 37.5 mg once daily—and adjusted only if needed.

But how do you know what dose of phentermine might be right for you, or whether you should try Qsymia^®? Let’s walk through how dosing works, the different strengths available, when Qsymia^® might be a better option, and what to expect when starting treatment.

How phentermine dosing works

Phentermine comes in a range of forms and doses, giving providers room to match the medication to your goals and tolerance. Whether it’s a single daily tablet or a smaller split dose, your treatment is tailored to fit your needs.

Most people begin with 15 mg to 37.5 mg once daily in the morning. The idea is to find the lowest effective dose that still supports appetite control while being well tolerated.

If you need an even lower dose, Lomaira™ comes in 8 mg tablets, which can be taken up to three times daily before meals. This lower-dose version was designed to provide more flexibility and help limit side effects while still offering appetite control.

The highest FDA-approved dose for phentermine is 37.5 mg once daily, and use is officially approved only for up to 12 weeks. Your doctor can prescribe it for longer use off-label.

Here are the available formulations for phentermine:

Dosage strengths and formulation	Availability
15 mg, 30 mg, 37.5 mg tablets or capsules	Generic
15 mg, 30 mg, 37.5 mg orally disintegrating tablets (ODTs)	Generic
8 mg tablets	Lomaira™ (brand only)

How does dosage work with Qsymia^®?

While phentermine can be prescribed for longer off-label, Qsymia^®, a combination of phentermine and topiramate, is FDA-approved for longer-term use. Qsymia^® follows a step-up schedule so your body can adjust gradually, starting with the lowest dose and increasing if needed. (Much the same way GLP-1 doses are increased.)

Here are the available strengths:

• 3.75 mg / 23 mg
• 7.5 mg / 46 mg
• 11.25 mg / 69 mg
• 15 mg / 92 mg

With Qsymia^®, you’ll begin at the lowest dose of 3.75 mg / 23 mg for the first two weeks. After that, your provider may raise it to the typical maintenance level of 7.5 mg / 46 mg. The dose can be increased further, from 11.25 mg / 69 mg up to the maximum dose of 15 mg / 92 mg, if more weight loss is needed. Each dose is taken once daily in the morning, and your provider will guide any changes based on your progress and side effects.

For a deeper breakdown, you can check out our full guide to Qsymia^® dosing.

Will costs increase as the dose increases?

Unlike many of the newer weight-loss medications, phentermine is relatively budget-friendly. Still, your cost can vary depending on the brand, your pharmacy, and your insurance coverage.

Here’s what current monthly prices typically look like without insurance:

• Generic phentermine (15 mg, 30 mg, or 37.5 mg tablets/capsules): $37–$41/month
• Lomaira^™ (8 mg tablets): around $28/month
• Qsymia^® (phentermine/topiramate ER): about $150/month
• Generic Qsymia^® (phentermine/topiramate ER): $60–$200/month

It’s not very common for weight-loss medications to be covered by insurance, but it does happen in some cases. It’s best to check directly with your provider or pharmacy to see what your options are.

Phentermine dosage: Adjustments & personalization

Phentermine dosing is always individualized, and the goal is to use the lowest dose that still works while keeping side effects manageable. Here’s how adjustments usually work:

• Side effects: If side effects like insomnia, restlessness, or dry mouth feel too strong, your provider may lower your dose or suggest splitting a 37.5 mg tablet into half doses. In some cases, they may also recommend switching to Lomaira^™, the 8 mg tablet taken three times daily. This approach allows smaller amounts spaced throughout the day, which may make side effects easier to tolerate.
• Medical factors: People with severe kidney impairment are limited to a maximum of 15 mg per day. Your provider will also consider any other health conditions, medications, or age-related concerns before changing your dose.

Every adjustment should be guided by your provider, not self-directed. If your treatment plan shifts toward longer-term use, your provider may also discuss alternatives such as Qsymia^® or GLP-1 medications.

How much weight can I lose with phentermine?

Phentermine is generally used as a short-term tool to help jumpstart weight loss. When combined with healthy eating and regular activity, a study shows it can help people lose an average of 5% of their starting weight over 12 weeks.

Your individual results may differ depending on your habits and how your body responds, but most people begin to notice changes within the first couple of weeks.

How to take phentermine: Tips for the best results

How and when you take phentermine can make a difference in both your results and how well you tolerate it. Consistency is key, and small adjustments to timing can also help manage side effects.

Action	Details
When to take it	Take phentermine once a day, usually in the morning before breakfast or 1 to 2 hours after. Avoid late-day doses, since the medication may interfere with sleep.
How to take it	Swallow tablets or capsules whole with water. If using the orally disintegrating tablet (ODT), let it dissolve on your tongue; no need for water. For Lomaira™ (8 mg), take up to three times daily, about 30 minutes before meals.
Splitting doses	Some providers may suggest splitting the 37.5 mg tablet in half (18.75 mg) to start more gradually or help reduce side effects.
Staying consistent	Try to take your dose at the same time each day so it becomes part of your routine. Pairing it with breakfast can help with consistency and reduce nausea.
How to minimize side effects	Take early in the day to avoid insomnia, sip water often for dry mouth, and eat smaller, balanced meals to reduce digestive discomfort.
Storage tips	Keep phentermine in its original container, tightly closed, and stored at room temperature away from heat and moisture. For unused pills, ask your pharmacist about take-back programs instead of flushing them.

What happens if you miss a dose?

If you miss a dose of phentermine, the general advice is to take it as soon as you remember. But if it’s already close to the time for your next scheduled dose, it’s better to skip the missed one and continue on your regular schedule. 

Doubling up on doses to “make up” for a missed pill can increase the risk of side effects such as insomnia, restlessness, or an elevated heart rate. If you’re unsure what to do or if you miss more than one dose, it’s always best to check in with your healthcare provider for personalized guidance.

Phentermine: Side effects and safety considerations

Phentermine can come with side effects, most of which are mild and temporary, especially in the first days or weeks, but some can be more serious and need quick medical attention.

Most common side effects of phentermine

The most common side effects of phentermine include dry mouth, unpleasant or metallic taste, constipation, diarrhea, nausea or vomiting, difficulty sleeping, restlessness, dizziness, tremor, and sometimes headaches. These effects are usually manageable, especially when you follow your provider’s guidance on timing and dosage.

Serious side effects and warnings

While most people taking phentermine experience only mild, temporary side effects, there are rare but more serious reactions that require immediate medical attention.

• Increased blood pressure and heart rate: Call your doctor if you notice a consistently fast heartbeat, palpitations, or elevated blood pressure that doesn’t go away.
• Chest pain or shortness of breath: Seek immediate care if you develop chest pain, trouble breathing, or swelling in the legs or ankles.
• Difficulty with exercise tolerance: If you feel unusually tired, get winded easily, or notice a drop in stamina with activities you could previously handle, let your provider know right away.
• Severe restlessness, agitation, or mood changes: Report extreme nervousness, irritability, or sudden mood changes promptly to your doctor.
• Insomnia or persistent trouble sleeping: If sleep problems interfere with daily life, your provider may need to adjust or discontinue your dose.
• Allergic reactions: Rash, itching, swelling, dizziness, or trouble breathing require urgent medical attention.
• Potential for misuse or dependence: Signs of dependence may include feeling unable to function without the medication, taking more than prescribed, or cravings for the drug. Contact your provider immediately if you notice any of these behaviors.

Always inform your doctor about any other medications or supplements you take. If you notice new or unusual symptoms, check in with your provider promptly. They can adjust your dose or explore alternatives if needed.

Who shouldn’t take phentermine?

Phentermine isn’t right for everyone. Before beginning treatment, inform your doctor if you have: 

• Heart problems: If you have a history of heart disease, uncontrolled high blood pressure, stroke, or irregular heart rhythms, you shouldn’t take phentermine.
• Eye conditions: Avoid phentermine if you have glaucoma, since it can increase eye pressure.
• Thyroid disorders: People with an overactive thyroid (hyperthyroidism) shouldn’t use phentermine.
• Recent MAOI use: If you have used a monoamine oxidase inhibitor (MAOI) antidepressant in the last 14 days, you shouldn’t take phentermine.
• Allergies to phentermine: People who are allergic to phentermine or any of its ingredients can’t take it. 
• People who are pregnant or breastfeeding also can’t take phentermine.

Overdose risks and symptoms

Phentermine should only be used exactly as prescribed. Taking excessive amounts can result in serious, potentially life-threatening effects. Both acute (sudden) and chronic (long-term) overdoses have distinct signs to watch for.

Symptoms of acute overdose may include:

• Restlessness, tremors, or shaking
• Rapid breathing or hyperactive reflexes
• Confusion, panic, hallucinations, or aggressive behavior
• Fatigue or depression after overstimulation
• Fast or irregular heartbeat, high or low blood pressure, or even circulatory collapse
• Nausea, vomiting, diarrhea, or abdominal cramps
• Severe cases can lead to convulsions or coma

Chronic overdose may show up as:

• Persistent insomnia and irritability
• Hyperactivity or personality changes
• Psychosis that can resemble schizophrenia
• Severe skin reactions

What to do if overdose is suspected:

Call the poison help line at 1-800-222-1222 right away. If the person collapses, has a seizure, has trouble breathing, or can’t be awakened, call 911 immediately.

The bottom line: Finding the phentermine dose for safety and results

Phentermine has stood the test of time as a weight loss aid, and understanding how doses are prescribed is one of the most important steps in using it safely. Whether it’s a standard 37.5 mg dose, a smaller split, or the lower-dose flexibility of Lomaira^™, your provider’s role is to help you stay on the lowest dose that works for your goals. 

If you’re considering long-term weight management, see if you qualify for Noom Med*. You’ll be connected with a clinician who can assess whether weight loss medication is right for you and prescribe one of several options if needed. Your medication treatment will be paired with Noom’s behavioral tools and expert coaching, creating a plan that supports both short-term progress and lasting change.

*Note: Phentermine or phentermine-containing products like Qsymia can only be prescribed by a physician you can see physically in a clinic and not through telehealth with Noom Med.