Phentermine promotes weight loss by triggering the release of hormones that suppress hunger signals and boost concentration and alertness.
However, as a result of this increased energy and alertness, many patients battle phentermine insomnia. When persistent, this common phentermine side effect can make getting enough sleep a frustratingly difficult task.
Phentermine functions as a central nervous system (CNS) stimulant, in many ways similar to amphetamine. It increases levels of a class of chemicals in the brain called catecholamines, which includes dopamine and norepinephrine (noradrenaline).
This chemical change produces a temporary but significant, boost in energy and decrease in appetite, which is what helps patients lose weight . Unfortunately, that same cascade can also cause difficulty sleeping.
Patients’ increased levels of energy and alertness often extend into the night and make it hard to fall asleep, even despite feelings of extreme tiredness.
Many patients experience insomnia during the first few days or weeks of treatment, but most people experience some relief as the body adjusts to phentermine.
If this side effect continues (e.g. you are still experiencing phentermine insomnia after 2 months), consult with your doctor. He or she may suggest a lower dose, amend your dosing schedule or suggest lifestyle modifications to help you overcome this side effect.
Contact your prescribing doctor if phentermine insomnia persists past the first week of treatment, grows more severe or interferes with your daily tasks.
Phentermine insomnia can keep you up for hours or even days at a time, despite intense feelings of tiredness. So many people wonder how to fall asleep on phentermine. There is no magic phentermine insomnia cure, but there are some common strategies that help phentermine insomnia go away.
If you find yourself battling recurrent insomnia, try these five simple tips to sleep when on phentermine:
Take phentermine first thing in the morning to reduce the risk of phentermine insomnia. Most patients take their pill on an empty stomach, either 30 minutes before or 1-2 hours after breakfast.
Phentermine reaches its maximum potency in the blood after about 1-3 hours and is completely absorbed in 4-6 hours, depending on physical activity and the amount of food in the stomach . However, most patients feel phentermine's energy-boosting effects for closer to 10-14 hours .
So, doctors and pharmacists recommend taking the last dose pill at least 8-10 hours before bedtime to minimize medication-related insomnia.
Excess caffeine consumption interferes with your ability to get enough restful, reparative sleep.
More, it's best to steer clear of caffeinated beverages while taking phentermine due to the medication's already-strong stimulant effects. Consuming caffeine while taking phentermine can cause or exacerbate adverse reactions like racing heart, chest pain or high blood pressure .
Avoid coffee, caffeinated tea, colas and energy drinks – all of which contain caffeine. Instead, opt for caffeine-free, sugar-free options like water, milk or herbal tea.
One way to burn off the extra energy from phentermine and reduce insomnia is to increase daily activity. This can come in the form of either formal workouts or simply moving more throughout the day.
Evidence indicates that getting your blood pumping first thing in the morning is best for overcoming insomnia. In fact, the National Sleep Foundation reports that, “people who work out on a treadmill at 7:00am sleep longer, experience deeper sleep cycles, and spend 75 percent more time in the most reparative stages of slumber than those who exercise at later times that day.” 
But don't worry: afternoon workouts also help combat insomnia. The only time of day that exercise does not help with insomnia from phentermine is late at night.
Aerobic exercise too close to bedtime increases core body temperature and signals your body not to sleep. If you have some extra energy right before bed, opt for a more relaxing exercise like yoga or stretching .
Research shows that too much screen time negatively affects users’ mental health and can interfere with restful sleep . So, try to disconnect as much as possible in the late evening hours by reducing use of smartphones, computers and televisions.
Keep your bedroom free of technological devices, work or other distractions. By designating room as a space for rest and relaxation you’re signaling to your brain that the bedroom is a place for sleep, not stress and wakefulness.
Take a moment to consider the physical environment in your room as well. Make sure your bedroom is a comfortable temperature for sleeping (preferably on the cooler side) and that light from the street or other rooms is kept to a minimum.
Many patients wonder, "Can you take sleeping pills with phentermine?". Unfortunately, it's not a simple yes or no answer
Some combinations are dangerous, while others can help you sleep and promote greater weight loss. However, sleeping pills are not a safe option for all patients so it's important that you speak directly with your prescribing physician and pharmacist.
Phentermine and melatonin are a common combination, especially for patients struggling with insomnia. However, the experts still don't know much about the specific drug interaction and some doctors warn about the two substances’ opposing effects.
Check with your personal doctor and pharmacist before combining phentermine and melatonin (or any other medication or supplement) to make sure it's safe in your particular case.
Click here to read more about supplements to take with phentermine.
Learn more about phentermine!
3. U.S. National Library of Medicine. (2006, January 12). PHENTERMINE.
4. Dr. Rick, MD. (2012, April 02). Phentermine 37.5 mg/daily in moring. My question is how does...
5. Drugs.com. (2019, January).Caffeine and Phentermine Drug Interactions.
6. National Sleep Foundation. (2018). What Time of Day to Exercise for Better Sleep.
7. Thomée, S., Härenstam, A., & Hagberg, M. (2012). Computer use and stress, sleep disturbances, and symptoms of depression among young adults – a prospective cohort study. BMC Psychiatry, 12(1). doi:10.1186/1471-244x-12-176