Best Supplements

Best Supplements to Take with Phentermine

Prescription appetite suppressants are potent on their own, but when phentermine stops working, adding a supplement is one way to keep the weight loss going.

Some doctors specifically instruct their patients to take supplements alongside phentermine, but most patients are left to investigate on their own, especially as they transition off the prescription medication.

Here are some of the most popular weight loss supplements:

Always ask your prescribing doctor and pharmacist before combining ANY additional medications or supplements with phentermine. 


Woman sleeping restfully in her bed after taking melatonin
Melatonin promotes more restful sleep, which makes it popular among patients who have phentermine insomnia

Many phentermine users are drawn to melatonin as a sleep aid if they struggle with phentermine-related insomnia. However, you should always ask your doctor before combining melatonin and phentermine, given the substances’ opposite effects.

Purpose of Melatonin​

Melatonin is a hormone that controls sleep-wake cycles. It’s made naturally in the brain’s pineal gland, but can also be taken as a supplement. Melatonin supplements are widely-available as pills, gummies or sub-lingual tabs, in doses ranging from 1-10 milligrams ( 1 ).

Melatonin supplements are commonly used to adjust sleep schedule changes associated with alternating shifts, jet lag, insomnia, or other sleep disorders. It works very well for some users, but others note little to no difference when using melatonin.

However, melatonin has several negative side effects, including daytime sleepiness, headaches, dizziness, stomach discomfort, and mood changes (anxiety, depression, or crankiness).

It can also cause problems when taken at the same time as other medications, such as: blood thinners, immunosuppressants, diabetes medications or birth control pills ( 2 ).

Melatonin and Phentermine

The interaction between phentermine and melatonin remains undefined. Some sources warn that phentermine increases alertness, while melatonin dampens it.

As a result, it is not advisable to mix melatonin and phentermine without a medical professional’s advice. However, some doctors do specifically prescribe this supplement to phentermine patients battling insomnia.

Ask your doctor and pharmacist if you want to take melatonin and phentermine together.

Capsicum / Cayenne Pepper

Red peppers
Capsicum comes from hot peppers and is thought to boost metabolism for weight loss

Food-based supplements have earned a certain fame in the past few years. One of the most popular, and most researched, food ingredient supplements is capsicum. It is a chemical derived from hot chili peppers, and studies show it accelerates metabolism to promote weight loss.

Purpose of Capsicum

Capsicum helps suppress appetite, curb cravings and boost metabolism. Research reveals that adding just a small amount of cayenne pepper to food helps people eat less and resist cravings.

The beneficial effects of this chemical are especially strong for people who don’t usually eat spicy food (3).

Capsicum, like any spicy food, also increases core body temperature. The human body likes to exist at a very exact temperature, so, in response to this spike, the metabolism revs and brings core temperature back down to normal. In this way, capsicum boosts metabolism and promotes weight loss.

In fact, subjects display higher resting energy expenditure for multiple hours after taking a capsicum supplement or eating cayenne pepper (3).

Fat Burners and Phentermine

Capsicum, in the form of cayenne pepper, is a popular ingredient in foods like curries, chili, and seasonings. It is also common in fat-burning dietary supplements.

There is no listed drug interaction between phentermine and capsicum, but that does not mean that an interaction cannot occur. Plus, other fat burner ingredients (like caffeine) may interact dangerously with phentermine. Capsicum or other ingredients may also interact with other medications. So, ALWAYS consult with your doctor and pharmacist before taking any medication or supplement alongside phentermine.

Pre-Workout Supplements

woman doing bicep curls
Do not combine phentermine and pre-workout supplement

Exercise is critical to weight loss success, but some people need some extra energy to power through a tough workout. Understandably, then, many patients wonder about combining phentermine with a pre-workout supplement to give their gym sessions an extra boost.

Purpose of Pre-Workout Supplements

Pre-workout supplements, commonly referred to as “pre-workout,” are powders that fitness enthusiasts take just before exercise to enhance performance.

Different brands use different ingredients, but popular components include quick-digesting carbs, amino acids, and B vitamins. Some products also contain caffeine or special ingredients like beet juice.

These supplements claim to improve performance by boosting energy, and in some cases accelerating metabolism or acting as a stimulant. However, experts consistently conclude that these products do not live up to their packaging claims in most cases ( 4 ).

Phentermine and Pre-Workout Supplement

Since phentermine already boosts energy by promoting a biological response similar to ‘fight or flight,’ taking a pre-workout supplement is just taking more of the same (and not in a good way). This combination could cause or exacerbate dangerous side effects like rapid heart rate or spikes in blood pressure.

You cannot predict how your body will react, and every pre-workout supplement is different, so taking phentermine and pre-workout together is not a good choice.

Do not take pre-workout supplements while taking phentermine.

Instead of Pre-Workout (Alternatives)​

Instead of risking the combination of phentermine and pre-workout, try these ideas to boost your energy before a big workout:

  • Try AM Workouts
    Schedule your workout around the time you feel the biggest energy boost from phentermine. Many people feel this rush within the first couple of hours after taking the pill, so consider exercising in that time period first thing in the morning.
  • Fuel for Success
    If you find yourself hitting a wall during workouts, ensure you’re properly nourished and hydrated beforehand. While taking phentermine, it is extra-important to stay on top of proper hydration and eat regularly to minimize unwanted side effects or crashes during workouts.

Vitamin B12

Vitamin B12 injections
Many clinics offer B12 injections to boost weight loss with phentermine

Weight loss doctors frequently prescribe B12 shots or pills with phentermine to boost patients’ energy and metabolism, but not all experts agree that this popular supplement serves any true purpose.

Purpose of B12 Supplements

B12 is a water-soluble vitamin found in most animal products, including eggs, fish, meat, and dairy. It is critical for the amino acid conversion that allows for RNA, DNA, protein, and lipid synthesis. It also supports proper neurological function and hemoglobin formation.

While B12 deficiency is relatively rare among healthy American adults, it is more common among older adults, vegans and people with gastrointestinal disorders ( 5 ).

As part of their standard slimdown program, many weight loss clinics offer B12 supplementation (either as a shot or a pill) to boost energy and metabolism. However, despite this common practice, one expert notes that extra B12 probably won’t help much for weight loss or energy unless you are deficient ( 6 ).

Phentermine and Vitamin B12 Pills

Many phentermine patients take phentermine and B12 together, but nearly always under the direct supervision of a doctor. While this vitamin is not likely to cause toxicity, it does interact with some medications.

Always tell your prescribing doctor about all of your current and recent medications and supplements.

If you are taking any medications for an acidy stomach, or metformin to manage diabetes, make sure to specifically discuss how these pills interact with B12.

Conjugated Linoleic Acid (CLA)

fit, muscular woman lifting weights
Some studies indicate CLA could help build muscle, but research remains inconclusive

CLA supplements are a common choice for phentermine users looking to boost their results on the scale, in the gym, and otherwise. Patients report promising results with this supplement, but science is mixed.

Purpose of CLA Supplements

Conjugated linoleic acid (CLA) supplements may help reduce body fat and weight when used in combination with regular physical activity. Scientific research is still mixed as to the effectiveness of this supplement, but many first-hand reports passionately support the use of CLA to reduce body fat.

Studies are also mixed regarding the supplement’s possible negative effects with long-term use, namely related to worsening lipid profiles or glucose metabolism ( 7 ). For this reason it is important to do your own research and assess whether CLA’s benefits outweigh its potential risks in your particular case.

Phentermine and CLA

This supplement may help boost weight loss, especially fat loss, during your phentermine journey. Some users feel it’s especially effective in increasing inches lost when used in combination with regular physical activity.

As always, tell your medical team about all medications and supplements in your daily routine. Check with your doctor and pharmacist before starting CLA.

5-Hydroxytryptophan (5-HTP)

woman measuring her waist
Combining 5 htp and phentermine may boost satiety and promote weight loss

5-HTP is one of the most common supplements for patients to take during or after their treatment with phentermine. It has been shown to increase satiety and boost weight loss on its own, but there is still no consensus as to whether it is safe or effective to combine 5-HTP with phentermine.

Purpose of 5-HTP​

5-hydroxytryptophan, or 5-HTP, is a precursor to serotonin. Serotonin helps control sleep, mood, and hunger, so low levels of this neurotransmitter can negatively affect health and well-being.

5-HTP supplements are made from either the seeds of the African tree called Griffonia or synthetically produced in chemical laboratories.

Most people consume adequate 5-HTP in their diets as long as they eat enough protein. However, stress, poor diet, or lack of exercise in our modern lifestyles can result in low levels of serotonin.

5-HTP promotes weight loss because it:

  • Increases feelings of fullness ( 8 )
  • Reduces stress-related eating ( 9 )
  • Alleviates insomnia, when combined with GABA ( 10 )
  • Boosts mood ( 11 )

5-HTP And Phentermine​

One observational study on combination pharmacotherapy for weight loss stated that,

“Combining 5-HTP and phentermine generally results in greater weight loss than would be observed with either medication alone.”

The author also noted that patients experience fewer stimulant side effects when phentermine and 5-HTP are administered at the same time. However, doctors agree that there need to be controlled trials before medical professionals adopt a widespread acceptance of this combination ( 12 ).

Like any other supplement, always consult with your doctor and pharmacist before combining 5-HTP and phentermine. A medical professional can help you navigate how much 5 HTP to take with phentermine and when to take 5 HTP (time of day) with phentermine.

Back to Phentermine Stopped Working

  1. National Sleep Foundation. (2018). What Is Melatonin? Retrieved from
  2. WebMD Staff. (2017). What Is Melatonin? Retrieved from WebMD.
  3. Ludy, M., & Mattes, R. D. (2011). The effects of hedonically acceptable red pepper doses on thermogenesis and appetite. Physiology & Behavior, 102(3-4), 251-258. doi:10.1016/j.physbeh.2010.11.018
  4. Fetters, K. A. (2017, Nov 12). What Are Pre-Workout Supplements — and Do You Need Them? Retrieved from
  5. Office of Dietary Supplements. (2018). Vitamin B12. Retrieved from National Institutes of Health: Office of Dietary Supplements.
  6. Bauer, D. B. (2018). Are vitamin B-12 injections helpful for weight loss? Retrieved from Mayo Clinic.
  7. Lehnen, T. E., Ramos da Silva, M., Camacho, A., Marcadenti, A., & Lehnen, A. M. (2015). A review on effects of conjugated linoleic fatty acid (CLA) upon body composition and energetic metabolism. Journal of the International Society of Sports Nutrition, 36.
  8. 5-hydroxytryptophan, eating behavior and adherence to dietary prescriptions in obese adult subjects. (1989). Clinical Nutrition, 8, 49. doi:10.1016/0261-5614(89)90158-1
  9. Amer, A., Breu, J., Mcdermott, J., Wurtman, R. J., & Maher, T. J. (2004). 5-Hydroxy-l-tryptophan suppresses food intake in food-deprived and stressed rats. Pharmacology Biochemistry and Behavior, 77(1), 137-143. doi:10.1016/j.pbb.2003.10.011
  10. Shell, W., Bullias, D., Charuvastra, E., May, L. A., & Silver, D. S. (2010). A Randomized, Placebo-Controlled Trial of an Amino Acid Preparation on Timing and Quality of Sleep. American Journal of Therapeutics, 17(2), 133-139. doi:10.1097/mjt.0b013e31819e9eab
  11. Shaw, K. A., Turner, J., & Mar, C. D. (2002). Tryptophan and 5-Hydroxytryptophan for depression. Cochrane Database of Systematic Reviews. doi:10.1002/14651858.cd003198
  12. Rothman, R. B. (2010). Treatment of Obesity With “Combination” Pharmacotherapy. American Journal of Therapeutics, 17(6), 596-603. doi:10.1097/mjt.0b013e31818e30da