What to Expect With Phentermine

Listen to this article on Spotify, watch it on YouTube, 或者用中文读。

Phentermine is a medication used very commonly for weight loss around the world. But how effective is it in practice, and what should you know before taking it?

Although phentermine is not available in Canada, it seems to be pretty widely available outside of Canada, and many of my patients from around the world have experiences using the medication and use the medication themselves during their weight loss.

So from all their experiences, I’m here to shed some light into what you might expect, and what you should know, if you try it yourself.

What is it?

Phentermine is a medication with many brand names, like Adipex, Duromine, Fastin, Lomaira, and many others.

It’s an amphetamine-like stimulant that has properties which cause appetite suppression.

Its approved use for weight loss dates all the way back to 1959, and has a long history of use, having a good safety and efficacy profile, compared to other weight loss drugs at the time.

Since it’s become generic, it’s also relatively cheap, contributing to its popularity worldwide, as it’s one of the most commonly used drugs for weight loss to date.

How does it work?

Phentermine works as an amphetamine-like appetite suppressant.

Amphetamine stimulants work by stimulating the sympathetic nervous system, also known as the fight-or-flight nervous system.

This system is normally activated during times of acute stress, and prepares your body for fight-or-flight by getting your muscles ready to act.

It also shuts down unnecessary systems in your body, like your digestive system. This shutdown results in a suppression of appetite.

The stimulant effects of phentermine also help you to feel more awake, allowing you to remain more physically active, and burn more fat off as a result.

Side effects of phentermine

From my experiences, sleep issues like insomnia seem to be pretty common. So most people end up taking phentermine earlier in the day, like mornings, to minimize sleep problems.

Gastrointestinal issues are also common, such as nausea, constipation or stomach pain.

In addition to suppressing your appetite, it may also modify the mouthfeel of some foods, by making foods taste strange or different, or causing your mouth to feel very dry.

From research studies, an outline of the common and rare side effects associated with phentermine is as follows:

Common side effects (reported in more than 1% of patients):

• Dry mouth (13-40%)
• Insomnia (8-24%)
• Headache (5-18%)
• Constipation (5-10%)
• Dizziness (3-10%)
• Diarrhea (2-10%)
• Nausea (2-8%)
• Increased heart rate (2-4%)
• Palpitations (2-4%)
• Restlessness (2-4%)
• Tingling sensation in extremities (2-4%)
• Unpleasant taste (2-4%)

Rare side effects (reported in less than 1% of patients):

• Cardiac arrest (frequency unknown)
• Hypertension (frequency unknown)
• Primary pulmonary hypertension (frequency unknown)
• Valvular heart disease (frequency unknown)
• Psychotic disorder (frequency unknown)
• Dependence and withdrawal symptoms (frequency unknown)

It’s important to note that phentermine can have an impact on the cardiovascular system and is contraindicated in patients with a history of cardiovascular disease or uncontrolled hypertension. Additionally, phentermine can be habit-forming and may cause withdrawal symptoms if abruptly discontinued.

You should be regularly screened by your clinician to check your heart rate and blood pressure, to make sure phentermine isn’t going to cause more serious cardiovascular issues down the road.

It can worsen anxiety in some people, so those with anxiety or mood disorders need to let their doctor know to see if it’s appropriate for them to use.

It also interacts with a number of medications, particularly other stimulant or mood medications. Let your prescribing doctor know if you’re taking other drugs.

Is phentermine addictive?

Phentermine is considered an amphetamine-like stimulant.

Amphetamine stimulants have a long history of having addictive qualities, and being on amphetamines for a long period of time can increase one’s dependence and cravings to continually use it.

Interestingly, the addictive qualities of phentermine do not appear to be as strong as other amphetamines, and patients who use phentermine for longer periods are at less risk of developing a dependence or craving for the drug than say Adderall.

So studies would suggest that while there is a potential risk of addiction since phentermine is like an amphetamine, it’s not as addictive as straight-up amphetamines.

From my experiences, there are definitely people who become addicted to phentermine.

Patients report that it’s related to feeling energetic and feeling great while on the drug, as well as having to stay on the drug to avoid weight regain.

Discontinuing the drug can result in uncomfortable headaches, fatigue, and weight rebound in many people, particularly in patients who have taken phentermine for longer periods of time, at larger doses.

Can I drink alcohol on phentermine?

It’s not recommended. Alcohol can sometimes worsen some of phentermine’s negative side effects when taken together, particularly if you have cardiovascular issues to begin with.

Another concern is from patients using phentermine, who may also be addicted to alcohol. Phentermine is a stimulant, and alcohol is a depressant.

Because both have opposing effects, people who use them both might need to consume more of both phentermine and alcohol to achieve the same effects.

This means that these patients will need to take larger and larger doses over time, and because both phentermine and alcohol can be addictive, it significantly increases the addictiveness of both drugs, and can make it harder for you to wean off both.

Can I have caffeine on phentermine?

It’s generally recommended you try not to take any other stimulants with phentermine, whether it be another stimulating drug, like fluoxetine, a stimulating supplement, like ginseng, or stimulating food, like a highly caffeinated energy drink, since phentermine is already a stimulant, and adding more stimulation could worsen stimulant-related side effects like anxiety, restlessness, and insomnia.

In practice, while it may be prudent to avoid the excessive intake of caffeine, like from RedBull, a normal coffee or espresso per day is not likely going to cause issues in most people.

Phentermine diet plan

There is no specific dietary plan you need to follow with phentermine. However it is often recommended you drink plenty of water since it might help you to manage some of phentermine’s side effects like dry mouth.

Phentermine is a pretty potent appetite suppressant, and some people on phentermine take the appetite suppression too far and undergo extreme diets like fasting for most of the day.

This is not a good idea.

Extreme diets are not sustainable and work only in the short-term. They frequently result in weight rebound once the diet is stopped, and can result in nutrional deficiencies that will make it harder for your body to burn fat.

Combine that with the fact that phentermine only works for a short period of time, and weight regain is almost a certainty.

This cycle of rapid weight loss followed by weight regain is known as yoyo dieting, and may cause additional health risks in addition to not helping you lose weight.

You should make sure your dietary plan has a good variety of macronutrients, like protein and fat, and micronutrients, like vitamins and minerals, for example by following a calorie controlled Mediterranean Diet.

How effective is phentermine?

Phentermine seems to be most effective in short-term weight loss. Many people who use it can expect anywhere from 5-10% weight loss in a few months.

The problem with phentermine however, is that this weight loss is frequently not sustained.

From my experiences, there are some patients that find phentermine stops working for them as quickly as just a few weeks of taking it. Weight loss tends to plateau, and in some cases rebound.

Often it occurs if a proper weight loss diet and good habits were not implemented, however its effects of suppressing appetite do tend to diminish quickly over time.

Many people increase their dose to counteract the weakening effects of phentermine, however, this only serves to delay the time when phentermine stops working for you.

How to properly use phentermine

Try to use phentermine it as a temporary support tool, for short periods of time, aiming to come off of it and have your weight loss diet take over as soon as possible.

You could also use it as needed, for example by taking a small maintenance dose during a vacation, and discontinuing it afterwards.

Developing and having a proper weight loss diet is imperative while on phentermine. Without one, rebound weight gain is almost a certainty.

You may also want to consider implementing an exercise routine if you haven’t already, since phentermine often helps improve energy for many people.

If phentermine stops working for you, and you’ve reached your maximum dosage, go back to your doctor first. They might recommend another medication or plan of action.

Don’t start increasing the dose yourself. Increasing your dose beyond the recommended dose and time you should be taking it, increases your risk of becoming addicted to phentermine and won’t make phentermine work any better.

There are patients that end up taking phentermine for years, because they’ve become addicted to it, even though it no longer helps them lose weight any further.

Should you try it?

Phentermine may be an option for you to help support weight loss, since it can produce effects quite quickly.

However, understand that these effects on your appetite and weight are strongest in the first few weeks, and are likely to grow weaker over time.

If you do decide to use it, keep in mind these few things:

1. Have a dietary and exercise plan ready to go before you start. Phentermine can start working pretty quickly and if you aren’t prepared, it could stop working before you have the chance to fully make use of it.
2. Get a prescription from a physician. Fake phentermine is rampant from non-prescription sources with supplements named things like “phentramine” or “phentermin” that contain no phentermine, but look and are named in a misleadingly similar way to phentermine.
3. Look to work with a clinician while on phentermine to monitor your weight loss progress and make sure phentermine is not causing you any health problems.

Learn more about

• Qsymia, a closely related newer weight loss drug
• 5 Things To Know Before Starting Weight Loss Medication
• Other weight loss medications

See also

• 
  Comparing Weight Loss Drugs in 2024
  Ozempic and Mounjaro are highly effective at treating obesity, however there are many other approved and off-label options that also work.
• 
  Bupropion vs Contrave vs Naltrexone
  Contrave is a synergistic combination of buproprion and naltrexone, which can be replicated to some degree using the generics individually.
• 
  New Obesity Drugs in 2024
  New obesity drugs target more than just GLP-1 and do much more than suppress appetite for a more nuanced approach to weight loss.
• 
  Mounjaro: Who Loses the Most Weight?
  White or Asian younger women who use metformin and have lower sugar and lipid levels tend to experience more weight loss with Mounjaro.
• 
  How does Oforglipron compare?
  Orforglipron, Semaglutide, and Danuglipron are oral GLP-1 agonists that are all similar enough that cost may end up being the winning factor.