Phentermine vs. Vyvanse: Differences, similarities, and side effects

In this article:

• Key differences
• Conditions treated
• Cost
• Side effects
• Drug interactions
• Drug warnings

In this article

• Key differences
• Conditions treated
• Cost
• Side effects
• Drug interactions
• Drug warnings

Phentermine and Vyvanse are two fairly different prescription drugs. They’re both central nervous system (CNS) stimulant drugs that work by increasing levels of certain neurotransmitters in the brain—but that’s where the similarities end. Phentermine is usually prescribed as a weight loss drug, while Vyvanse is prescribed for attention-deficit hyperactivity disorder (ADHD) and binge-eating disorder. These medications also have different side effects, costs, interactions, and more. 

Phentermine vs. Vyvanse: Key differences

Vyvanse is a brand-name medication that is available in tablet or capsule form, and it is also available as the generic lisdexamfetamine dimesylate. It is a stimulant that’s approved by the Food and Drug Administration (FDA) to treat ADHD in adults and children 6 years and older. It’s also the only FDA-approved medication for treating binge eating disorder in adults. 

Vyvanse reduces symptoms of impulsivity and inattention. “Its primary mechanism is to increase the levels of certain neurotransmitters in the brain—dopamine and norepinephrine. This action helps improve attention and focus, and can reduce impulsivity,” says Danielle Kelvas, MD, primary care physician at The HCG Institute and chief medical editor at DKMD Consulting. In many cases, it’s prescribed as a longer-term maintenance drug.

Phentermine is a generic drug sold in tablet or capsule form. It’s available under the brand names Adipex-P and Lomaira, and it’s also combined with the anticonvulsant topiramate in a brand-name drug called Qsymia. Phentermine is “a sympathomimetic amine with pharmacologic activity similar to that of amphetamines,” Dr. Kelvas says. “It suppresses appetite and possibly increases energy expenditure by interacting with the central nervous system.” phentermine is typically only prescribed for short-term use, although some healthcare providers may also approve it for longer treatments.

It’s worth noting that both medications are classified as controlled substances by the Drug Enforcement Administration. Phentermine is Schedule IV and Vyvanse is Schedule II, which means Vyvanse has a higher risk of abuse and dependence. That’s why Dr. Kelvas says, “It’s critical for drugs like this (addictive controlled stimulants) to be given in tandem with cognitive behavioral therapy, or some sort of regular ongoing follow-up.” Many states have additional requirements for physicians to prescribe Scheduled II, III, and VI medications.  These requirements may include the patient signing a controlled substance agreement, having the patient perform an initial (and random) urine drug screen, and assess the risk for misuse or abuse.  

Phentermine vs. Vyvanse: Conditions treated

Vyvanse is FDA approved to treat ADHD and binge-eating disorder, and studies have confirmed its effectiveness in improving adult ADHD and reducing binge-eating episodes. Healthcare providers may also prescribe it off-label to treat depression or narcolepsy. “While Vyvanse can help those with binge eating disorder lose weight, it’s not a weight loss drug. However, some people may abuse it to lose weight, as weight loss can occur with Vyvanse use,” says Michael McGrath, MD, board-certified psychiatrist and chief clinical officer at The Ohana Hawaii. “There are much safer, and FDA-approved medications for weight loss.”

Phentermine is only FDA approved for short-term weight reduction, and modern research has proven its efficacy (especially when combined with topiramate). In most cases, it works best alongside lifestyle changes like diet and exercise. “Phentermine is not approved by the FDA to treat ADHD but it’s sometimes prescribed off-label for ADHD,” says Dr. McGrath. “This drug works similarly to ADHD medications, which is why it’s prescribed off-label for this. However, the research is scarce on the effectiveness of phentermine for ADHD.”

Phentermine vs. Vyvanse cost 

As a generic drug, phentermine often costs significantly less than brand-name Vyvanse. For people paying completely out of pocket, phentermine can cost around $51 for a 30-day supply, while Vyvanse costs around $539.

Of course, not everyone has to pay 100% out of pocket. Some insurance and Medicare Part D plans will cover Vyvanse when it’s deemed medically necessary—although some might have prior authorization requirements. Many insurance and Medicare plans, however, refuse to cover weight loss drugs, so phentermine might not receive coverage. It all depends on your specific plan, so check your formulary to find out for sure.

Yet insurance coverage isn’t the only way to save on phentermine and Vyvanse. Coupons from SingleCare can significantly reduce the price of either drug. It’s free to sign up and you just need to present your discount card at a local participating pharmacy to reap the benefits.

*Cost at time of publication. Drug prices fluctuate and vary by location. 

Phentermine vs. Vyvanse side effects

The most common side effects of Vyvanse for adults with ADHD are decreased appetite, insomnia, dry mouth, and irritability. For adults with binge-eating disorder, the most common side effects are dry mouth, insomnia, decreased appetite, and increased heart rate. Abusing Vyvanse can also cause heart arrhythmia and psychosis, according to Dr. McGrath.

The side effects of phentermine include irregular heartbeat, elevated blood pressure, restlessness, dizziness, and insomnia. And per Dr. McGrath, “Both phentermine and Vyvanse have side effects of dry mouth, nausea, and vomiting.”

In rare cases, each drug can also cause serious side effects. For example, phentermine may cause cardiovascular issues like pulmonary hypertension, rapid heart rate, and heart failure. Vyvanse can cause similar heart issues, plus serotonin syndrome, seizures, anxiety, and more. Dr. Kelvas says she also often has patients report palpitations—sometimes to the point that the patient needs to discontinue Vyvanse treatment.

These are only some of the possible side effects. Talk to your healthcare provider about how either drug may affect you. 

Source: Phentermine label and Vyvanse label

Phentermine and Vyvanse drug interactions

The most serious Vyvanse drug interaction occurs when Vyvanse is taken with monoamine oxidase inhibitors (MAOIs). Taking them together can cause life-threatening blood pressure spikes, so patients should wait 14 days after taking their last MAOI before taking phentermine. It can also cause seizures when mixed with some antipsychotics (or bupropion) and serotonin syndrome when combined with certain selective serotonin reuptake inhibitors (SSRIs).

Many of the same drugs can interact with Vyvanse. It has the same serious interaction with MAOIs, and drugs that affect serotonin—like SSRIs, SNRIs, tricyclic antidepressants, and some headache drugs. Other CNS stimulants can further push the heart rate and blood pressure to dangerous levels when combined with Vyvanse, and some analyzing agents can reduce the body’s ability to eliminate the drug.

*This may not be a complete list of drug interactions for phentermine and Vyvanse. Additionally, the likeness or severity of these interactions may depend on the patient’s medical condition. Always consult a healthcare provider before combining drugs. 

Phentermine and Vyvanse drug warnings

Patients who should not use Vyvanse include those who have preexisting heart conditions like high blood pressure (since it can cause cardiovascular reactions), a medical history of substance abuse, hypersensitivity to amphetamines, or those who have taken an MAOI within 14 days. On the other hand, phentermine’s label says phentermine use is contraindicated for people who have a history of heart disease, hyperthyroidism, glaucoma, drug abuse, or those who have taken an MAOI within 14 days.

People who are pregnant or breastfeeding should avoid both drugs because they may harm the fetus and it’s unclear how much of the drugs are passed through breast milk.

As we’ve already mentioned, both drugs can lead to abuse or physical and psychological dependence. They can also cause withdrawal when a patient stops using them.

Bottom line

Phentermine and Vyvanse might affect the body in similar ways—specifically when it comes to appetite—but they’re very different drugs with distinct uses. While phentermine is a low-cost, short-term, generic option for people looking to lose weight, Vyvanse is focused primarily on ADHD and binge-eating disorder treatment and tends to be more expensive. Regardless, a healthcare provider can provide specific medical advice on which might be best and safest for a particular patient’s condition.