Can You Drink Caffeine While Taking Phentermine?

Phentermine is a weight loss medication from the psychostimulant family of drugs. While no specific interactions exist between caffeine and phentermine, combining the two can lead to unwanted side effects. Understanding how caffeine interacts with phentermine allows you to use both substances more safely. To prevent harmful outcomes, consult your physician before using phentermine with caffeine.


Phentermine is an amphetaminelike medication prescribed as a temporary weight loss aid. When combined with a low-calorie diet and regular exercise, phentermine helps jumpstart weight loss by reducing your appetite and speeding metabolism. It belongs to a family of drugs known as anorectic stimulants. Phentermine is available as tablets and extended-release capsules and is generally prescribed for short periods of three to six weeks. Caffeine is also a mild central nervous system stimulant that shares some effects with phentermine. It can be found in many over-the-counter items such as energy drinks, coffee and tea.

Side Effects/Interactions

Phentermine can cause unwanted effects in some users. MedlinePlus lists insomnia, increased blood pressure, restlessness and heart palpitations as possible side effects. Combining phentermine with other stimulants can increase the risk of developing certain side effects. According to Drug Information Online, caffeine and phentermine are not specifically contraindicated. Still, taking the two substances together may result in overstimulation due to their similar effects on the cardiovascular system and brain.


Limiting your caffeine consumption while taking phentermine can help prevent unwanted side effects. Talk to your doctor about the safety of combining phentermine and caffeine, especially if you have high blood pressure or another cardiovascular condition. Substituting drinks such as decaffeinated coffee and herbal tea for caffeinated beverages can help reduce your daily caffeine consumption. Since caffeine is present in many foods, medications and supplements, you may need to read ingredient labels to determine which items contain the substance. Monitoring your blood pressure and other vital signs after consuming phentermine and caffeine can help you determine how they affect your body. Stop using phentermine and seek prompt medical attention if you develop chest pain, rapid heartbeat or other serious side effects.


The effects of caffeine differ from person to person. Not all people who combine caffeine and phentermine will experience adverse reactions. While small amounts are not usually harmful, you may need to avoid caffeine altogether if you're sensitive to its effects.

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