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If you are someone who takes phentermine for weight management, you might be wondering if it’s okay to have a cup of coffee every now and then. How about tea, soda, or other forms of caffeine? After all, phentermine is a stimulant and can have some of the same side effects as caffeine, such as rapid heartbeat and jitteriness. Is it possible that combining phentermine and coffee will have some unpleasant—or even harmful—side effects?
Let’s take a look at what happens when you mix phentermine and caffeine, and whether you can drink caffeine while taking phentermine.
What is phentermine?
Phentermine is a medication that can suppress your appetite. It’s often used to help overweight or obese people who are having trouble losing weight. The medication is usually combined with other weight loss strategies such as calorie reduction and exercise. If you’re wondering how much caffeine is in phentermine, the answer is zero. Phentermine doesn’t have caffeine in it, but like caffeine, it’s a stimulant, and has many of the same side effects as caffeine.
RELATED: Is phentermine for weight loss safe?
Do phentermine and caffeine interact?
There is little published data out there to answer questions like, “Can you drink coffee with phentermine?” But in theory, phentermine and caffeine could interact with each other, says Laura Purdy, MD, a board-certified family medicine physician.
The reason for this, Dr. Purdy says, is because both phentermine and caffeine stimulate the nervous system, and if combined together, you could potentially experience intense side effects. “I would expect that someone who is using phentermine and also chooses to take caffeine will have a very high blood pressure and heart rate, likely palpitations, have trouble with sleeping, and will likely have racing thoughts and a dry mouth,” Dr. Purdy explains.
Does caffeine interact with topiramate?
Phentermine is often combined with topiramate to maximize effects in people looking to lose weight. In 2012, the FDA approved a combination drug called Qsymia, which contains phentermine and topiramate, to be used as an appetite suppressant. As such, if you are taking topiramate along with phentermine, you may be wondering if you can consume caffeine.
According to Erika Gray, Pharm.D., chief medical officer and co-founder of Toolbox Genomics, there is some research indicating caffeine interacts with topiramate by making it less effective. “This is especially problematic if you take topiramate for seizures,” she says. (Topamax, a single-ingredient drug that contains the active ingredient topiramate, is used to treat certain types of seizures.) However, she notes that there isn’t any research showing that caffeine decreases the effectiveness of topiramate when it comes to appetite suppression.
How much caffeine can you have with phentermine?
There are no set rules about how much caffeine you can consume with phentermine, says Dr. Purdy. Still, she recommends using precautions. “I would really advise that people drastically cut back, and they probably won’t even feel like they need caffeine when they are taking phentermine,” she explains.
It’s also important to remember that not all caffeine sources have the same amount of caffeine in them. “A Starbucks quadruple espresso has much more caffeine than a cup of green tea,” says Dr. Purdy. It’s possible there is a dose dependent impact when it comes to mixing phentermine and caffeine. “The more that you take, the more symptoms of caffeine excess or stimulant excess the person may have,” she explains.
How does caffeine affect people on appetite suppressants?
Phentermine isn’t the only stimulant appetite suppressant medication out there. The FDA has approved other stimulant medications for weight management, such as:
Appetite suppressants are meant to be used along with exercise and diet to help you lose weight, explains Sabrina Barata, MD, a primary care doctor at Mercy Personal Physicians at Lutherville in Maryland. “They help you suppress your appetite by stimulating your central nervous system and therefore reducing your appetite,” she says. “These medications essentially function as a stimulant.”
Again, the problem here is that caffeine is also a stimulant, which is why caffeine should be taken with caution anytime you are on a stimulant appetite suppressant, says Dr. Barata. “You should discuss with your doctor prior to initiation of any appetite suppressant in regards to any dietary modifications and caffeine use,” she recommends.
Any excessive caffeine use can lead to increased heart rate and elevated blood pressure, and people with medical conditions like thyroid disease, heart disease, glaucoma, and a history of seizures may be most vulnerable to these effects, Dr. Barata adds.
Note: There are other medications used for weight loss, such as Contrave (naltrexone/bupropion), Wegovy (semaglutide), or Saxenda (liraglutide). These drugs work differently than phentermine and are not stimulants. However, Contrave and caffeine can interact, increasing the risk of high blood pressure.
Best practices for taking phentermine
For best results, you should follow the specific instructions for the type of phentermine you are taking. Phentermine is often taken as a single dose first thing in the morning. It also may be taken as several doses throughout the day, usually about 30 minutes before a meal. Because side effects of phentermine include wakefulness and insomnia, it’s recommended that you take phentermine in the morning, and not at bedtime.
Anytime you are taking a new medication, you should discuss with your healthcare provider how it may interact with substances like caffeine, along with any other substances, foods, medications (Rx or OTC), or vitamins you may be consuming.
When it comes to phentermine, there aren’t any foods or vitamins that are known to have harmful side effects, explains Dr. Gray. But she says it’s always sensible to be cautious around substances like alcohol. “I always exercise extreme caution with my patients who consume alcohol and any medications—phentermine is no exception,” Dr. Gray offers.
Dr. Barata says that your healthcare provider may have specific recommendations about what foods to consume while on phentermine as you aim to reach your goal of losing weight in a safe and steady manner. “Your doctor may mention to avoid certain foods in regards to your diet meal plan while on phentermine to avoid changes in your blood sugar level, which may affect your overall weight loss, excessive fat storage, and therefore decreasing effectiveness of phentermine,” she says.
For example, simple carbohydrates and sugary, processed foods may be advised against, because of how they may affect your sugar levels, Dr. Barata says. It should also be noted that constipation is a common side effect of phentermine, so you should be sure to increase your fiber intake while taking it, as well as drink plenty of water. If you become seriously constipated, talk to your healthcare provider about using stool softeners, Dr. Barata recommends.
The recommendations about what to consume and what not to consume while taking phentermine will vary from one person to another, based on their medical history, and nutritional needs. If you have any further questions about mixing phentermine with caffeine—or any other dietary considerations while taking this medication—be sure to reach out to your healthcare provider.
