My Phentermine Stopped Working—Now What?

If you’ve struggled to cut calories, it’s exciting to start appetite suppressants like phentermine and see stubborn pounds finally melt away. But what if the scale slows down or stops moving altogether while you’re still taking the medication? 

This is a common concern for people who take phentermine, says obesity medicine physician Ethan Lazarus, M.D. Several Reddit threads show people who experienced either a weight plateau or regain after just a few weeks (or months) of treatment. Others report that their usual appetite came back and that they no longer experience the energy boost they did at the beginning of treatment.

What gives? Does phentermine really just stop working? 

About the Experts

Ethan Lazarus, M.D., is a board-certified family and obesity medicine physician and a former president and current delegate of the Obesity Medicine Association. 

Caroline Messer, M.D., is an endocrinologist with board certification in internal medicine, diabetes, and metabolism. She specializes in treating metabolic disorders and obesity.

What Is Phentermine?

Phentermine is a prescription appetite suppressant offered under the brand name Adipex-P, as a generic or as part of a drug like Qysmia, which is a combination of weight loss-promoting ingredients.

“Phentermine acts as a mild stimulant,” Lazarus explains. When you take a stimulant, you may feel an increase in attention and energy and a decrease in the desire to eat. You can think of phentermine as an “on-off switch” for your appetite. 

“From a biological perspective, a mild stimulant is telling your brain that it’s not time to eat—it’s time to run away from the rabid dog,” he adds. 

Phentermine is one of the oldest weight loss medications on the market, first hitting pharmacies in 1959. While doctors used to prescribe it short-term to help people drop weight quickly, phentermine is now used as a long-term weight loss treatment, endocrinologist Caroline Messer, M.D., says.

“After Qsymia, which contains phentermine and topiramate, was approved for long-term use, doctors began to rethink dosing strategies for phentermine,” she explains. Instead of short-term weight loss treatment—which people were using to slim down quickly—doctors are now using phentermine long-term to treat obesity as a chronic disease.

“Short-term weight loss makes very little sense for overall health,” Lazarus says. 

So, how is phentermine different from popular GLP-1 medications, including Ozempic and Mounjaro, that also shrink your appetite?

“Both phentermine and GLP-1 medications suppress appetite, but they work on opposite ends of the eating pathway,” Lazarus says. He describes phentermine as “a stop sign.”

“It tells you not to eat, so you won’t feel hungry,” he explains. This effect will make you less inclined to reach for snacks between meals or overindulge. 

GLP-1 drugs mimic the satisfaction hormone and slow the speed at which food leaves the stomach, so you stay fuller longer. So, unlike phentermine, which can make you not want to eat at all, a GLP-1 agonist will make you feel full more quickly after a meal, Lazarus says. 

Why Does Phentermine Stop Working For Some People?

Although phentermine is now prescribed for long-term use, some people may stop losing weight or feel like the stimulant effect of the medication has worn off. Messer and Lazarus shed some light on why this happens and whether or not it’s actually a cause for concern. 

They point to the idea of “phentermine tolerance,” which they believe is greatly misunderstood. The fact is that appetite suppressants generally result in less weight loss than other weight loss drugs (namely GLP-1 agonists).

“I see that [phentermine] helps patients lose approximately 10 to 15 pounds and then helps them maintain that weight loss,” endocrinologist Messer says. It does not generally help them lose more. “This is not a good medication for someone who needs to lose a bunch of weight,” she adds. 

Your body is fighting back

Turns out, your body doesn’t want to lose weight, thanks to a very necessary, but sometimes inconvenient, survival mechanism, Lazarus says.

“When you start to lose weight, the body fights back through adaptive thermogenesis,” he explains. 

Adaptive thermogenesis is the process by which your body changes the amount of energy it uses in response to things like diet, temperature, and physical activity (1). It helps your body maintain its current weight by adjusting how many calories you burn. This slows down your metabolism and makes you feel hungry if it senses you’re shedding pounds quickly. 

Over time, adaptive thermogenesis makes it seem like your treatment isn’t working as well as it was at the beginning, Lazarus says. But that’s not the full picture. “It’s not that you’ve acclimated to the drug or that the drug stopped working, it’s that your body is pushing back harder than the medication can suppress it.”

You started at a high dose

Lazarus notes that he sometimes sees non-obesity physicians prescribing high doses to patients from the get-go. While this may produce quicker and more dramatic weight loss results, it can also lead to less sustainable treatment in the long run. 

“We see a lot of people getting started on phentermine at the highest possible dose, which is 37.5 milligrams,” Lazarus says. “That’s like giving a patient 10 Starbucks coffees on day one—no wonder they report a massive energy boost at first.” 

While the stimulant effect of phentermine can feel like you chugged an energy drink, this energy burst is actually not a targeted benefit of treatment, he explains. “We’re trying to use phentermine to help with weight loss by lowering appetite, not to give people an energy burst.”

And even though adaptive thermogenesis is most likely to blame for slowed weight loss on phentermine, it’s also possible for your body to get used to the treatment. So if you start at the highest dose, you can’t safely increase it if your appetite starts to sneak back, Lazarus says. Instead, many obesity physicians will give people a splittable tablet and tell them to start with half, or around an eight-milligram dose of phentermine, he explains. This gives you plenty of room to titrate up if your doctor sees fit.

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You’ve maxed out weight loss potential

If you’ve heard stories about people beating food addiction and losing hundreds of pounds after starting other weight loss treatments, you might be under the impression that phentermine can give you the same results. However, there is a known limit to how much weight patients lose with this medication, Messer notes.

It’s understandable to have high expectations about starting an appetite suppressant if you’ve struggled to restrict calories. But Lazarus emphasizes that each weight loss medication can produce different results. 

“We need to not be thinking about goal weights,” he says. “Instead, we need to think about reducing the weight by a reasonable percentage from where it is now.” 

For people on phentermine, research suggests that a 10 percent weight loss is reasonable and healthy (2). So if you started at around 200 pounds, that’s about a 20-pound loss. 

If you have more than 20 pounds to lose to reach a healthy weight, Lazarus says he would likely prescribe a GLP-1 agonist or a drug like Contrave over phentermine. 

What to Do if Phentermine Stops Working

Good news, you’re not SOL if the appetite suppressant no longer serves you. The following steps can get the numbers on the scale moving again.

Discuss lifestyle tweaks

While phentermine can certainly help kickstart your weight loss, you’ll also need to make adjustments to your lifestyle to see the best possible results.

Lazarus says a successful phentermine program includes frequent exercise and eating a balanced diet within your daily recommended calories, which your doctor can help you calculate. 

Ask about other medications

Again, phentermine can help you achieve and sustain a modest 10- to 12-percent weight loss. A plateau doesn’t mean it’s stopped working. Although you may notice that you’re no longer losing weight, the sign that phentermine is no longer working is if you regain a significant amount back—and quickly.

“If patients are starting to gain back weight after a six-month phentermine treatment, it’s probably not the correct medication for them,” Messer says. If you’re regaining the weight you lost on phentermine, talk to your doctor about whether other weight loss medications could be a better fit. 

If weight loss simply plateaus, however, she encourages people to stick with the medication because continued use can help you maintain the weight loss you’ve already achieved. 

Can I take a break from phentermine?

While Lazarus says you may feel heightened energy and greater appetite suppression if you take a few weeks off of phentermine and then go back on it, that won’t move the needle on the scale if you’ve plateaued.

“In the old days we recommended tolerance breaks a fair amount,” Lazarus says. “But generally speaking, I don’t think it will give you better results. Even if you take a break, you may experience weight gain or hit another plateau.”

But if you’re set on taking a break, is it safe? If you’re on phentermine on its own, it’s completely fine to quit cold turkey, Messer says. One older, small-scale study suggests it’s rare to experience withdrawal symptoms after long-term phentermine, meaning you can safely stop the drug without weaning off of it (3). If you’re on Qsymia, you’ll need to wean yourself off with your doctor’s help. 

Just a heads up: Once you stop phentermine, your normal appetite may return. To help offset potential regain, stick to those healthy habits you adopted during your treatment, such as engaging in regular exercise and eating a diet with plenty of satiating fiber and protein. Talk to your doctor about whether or not stopping phentermine could put you at risk of regain and/or temporary side effects like fatigue.