Home > Side EffectsPhentermine Side EffectsPhentermine is the most-prescribed weight loss medication in the United States, but it’s not the perfect pill ( 1 ).The popular stimulant produces powerful appetite suppressing and energy boosting effects which, when used in combination with healthy diet and regular physical activity, encourage weight loss.However, in addition to these desirable reactions, phentermine also produces a wide range of unpleasant side effects. The severity of these reactions ranges from mild to life-threatening, so it’s important to understand the risks before undergoing treatment with this medication.Side Effects of Phentermine Many patients deal with phentermine side effects, including headacheMost patients experience at least one phentermine side effect while undergoing treatment. Dry mouth (xerostomia) and trouble sleeping (insomnia) are the most common reactions, but each patient is different.Severe Side EffectsPhentermine side effect dangers are primarily related to a handful of serious, potentially life-threatening reactions to this medication. Primary pulmonary hypertension (PPH) and heart valve irregularities are two very serious phentermine side effects.These complications were primarily observed in a group of patients that took the once-popular combination of phentermine and fenfluramine (colloquially referred to as “phen-fen”) in mid-1990s ( 2 ). While fenfluramine was ultimately deemed responsible for these serious complications, phentermine’s potentially-contributory role could never be ruled-out. As a result, these reactions are still listed on all phentermine labels. Click here to read more about phentermine heart problems.Other serious phentermine side effects relate to mood changes, metabolic imbalances, drug reactions and allergies.Symptoms of severe phentermine side effects include ( 3 , 4 ):Chest painPounding or irregular heartbeat, or fluttering in your chestLightheadedness or faintnessShortness of breath, even with only mild exertionSwelling in your legs, ankles or feet (edema)Slurred speech or weakness on one side of your bodyUnusual thoughts or behavior, such as confusion, irritability, phen rage or hallucinationsExtreme happiness or sadness (euphoria or phentermine depression)Any indicators of dangerously-high blood pressure, such as: severe headache, blurred vision, buzzing in your ears, anxiety, confusion, chest pain, shortness of breath, uneven heartbeats or seizure.Call 911 (emergency medical services) or go to a hospital right away if you notice any of these symptoms while taking phentermine.Note that this is not a complete list of side effects. When in doubt, always contact your doctor.Common Side EffectsThankfully, serious side effects are relatively rare. The majority of phentermine users experience reactions that are bothersome, but not typically considered dangerous.Common phentermine side effects include ( 5 , 6 , 7 , 8 ):Dry mouth or unpleasant tasteInsomniaConstipationHeadacheFatigueSexual side effectsMenstrual changesHair lossNausea or stomach painRashAcneDiarrheaThe most common phentermine side effects are dry mouth and trouble sleeping ( 9 ). To that end, a recent online poll of over 2100 phentermine users found that more than 4 in 5 had at least one instance of dry mouth while taking this medication ( 10 ). Results of a 2019 poll on users’ most common phentermine side effectsSide Effects of Qsymia (Phentermine and Topiramate)Pairing phentermine and topiramate – either as two separate pills or in the popular combined pill, Qsymia – carries an additional risk.Serious side effects of Qsymia include ( 11 ):Birth defects (cleft lip/palate), if a woman gets pregnant while taking this medicationIncreased heart rate or blood pressureSuicidal thoughts or actionsSerious eye problems (secondary angle closure glaucoma)Increased acid in the bloodstream (metabolic acidosis)Problems with memory, speech or concentrationSeizures when medication is stopped suddenlyKidney stonesDecreased sweating/increased body temperature (fever)Low blood sugar (hypoglycemia) in patients with type 2 diabetesLow blood sugar (hypoglycemia) in patients with type 2 diabetesGeneral central nervous system (CNS) side effectsDo Phentermine Side Effects Go Away? Phentermine side effects eventually go away for most usersYes – phentermine side effects are usually worst during the first days or week of treatment.In most patients, the body starts to return to normal with continued use of the medication. However, certain reactions such as dry mouth, constipation or increased heart rate may last for longer.Thankfully, it’s relatively rare for patients to experience phentermine long term side effects. Hair loss is the most commonly-reported long term reaction, but it is still not listed as an official side effect of the medication.Contact your doctor if any phentermine side effect does not go away, worsens or interferes with your daily life.How Long Can You Take Phentermine Safely? Phentermine is a short-term weight loss medicationPhentermine side effects can affect any patient at any time, but some factors make adverse reactions more likely.Taking phentermine for longer than recommended or prescribed, for example, increases the chances of adverse reactions.Phentermine is a short-term weight management tool ( 12 ). More specifically, the Food and Drug Administration (FDA) has only approved this medication for “a few weeks” of use, which is generally interpreted to mean no more than 12 weeks at a time ( 12 , 13 ).It is important to note, however, that this three-month limit is universal for all weight loss drugs approved prior to 1985, related to an old-fashioned belief that lifestyle habits could be modified in 12 weeks’ time ( 1 ). Talk to your doctor if you think you would benefit from a longer-term solution.How to Combat Side Effects of Phentermine Staying properly hydrated can help combat some phentermine side effectsEach phentermine side effect is treated differently depending on the timing, severity and nature of the reaction. Click on the links above to read more about each individual side effect and how to minimize it.However, if you are worried about phentermine side effects, make sure to discuss your concerns with the prescribing doctor. He or she can offer advice about strategies to minimize or avoid common reactions to this medication.Here are a few common suggestions to reduce phentermine side effects:Use the lowest effective doseDrink plenty of waterAvoid caffeine and alcoholInclude physical activity as permitted and ableTell your doctor about ANY AND ALL other medications and supplementsWork with your doctor to optimize dosage scheduleReport any worrisome symptoms to your doctor right awayALWAYS contact your doctor or seek professional medical attention if you’re concerned about a phentermine side effect, especially if is not going away, worsening or impacting your daily life.Learn more about phentermine!Phentermine DosagePhentermine Stopped WorkingPhentermine WithdrawalWhat Phentermine Looks LikeReferencesHendricks, E. J., Rothman, R. B., & Greenway, F. L. (2009). How Physician Obesity Specialists Use Drugs to Treat Obesity. Obesity, 17(9), 1730-1735. doi:10.1038/oby.2009.69Connolly, H. M., Crary, J. 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