Can Phentermine Cause Headaches?

Phentermine, commonly combined with Topiramate, is a weight loss drug used since the 1960s. It acts similarly to weight loss peptides by reducing appetite and stimulating weight loss. This comes at the price of side effects. This blog discusses whether phentermine causes headaches and other side effects you can expect from taking phentermine.
Klara Hatinova

Klara Hatinova

Klara is postgraduate researcher in experimental psychology at the
University of Oxford.

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Can Phentermine Cause Headaches?

Yes, phentermine can cause headaches. This is one of the common side effects of this medication. It's essential to consult with your healthcare provider if you experience severe or persistent headaches while taking phentermine.


Phentermine is a prescription drug that stimulates the effects of weight loss peptides—short proteins that can help reduce weight. It is available as a generic drug and a branded slow-release capsule [1].

Phentermine is commonly used in a fixed-dose combination with topiramate to treat obesity, sleep apnea syndrome, type 2 diabetes mellitus, and non-alcoholic steatohepatitis [2]. This combination has a lower risk of side effects because topiramate counteracts some of phentermine’s adverse effects [3].

Phentermine is a controlled substance and is closely monitored in medical care and by doping agencies. This is due to the effects of phentermine on the dopaminergic and adrenalin system, which make it easy to develop dependence. It is typically used short-term (for a few weeks) to treat obesity in very obese and overweight people.

In obesity management, phentermine is used in conjunction with behavioural modifications, such as exercise and a reduced-calorie diet. Weight loss does not occur without these [1].

It's important to note that phentermine's effectiveness decreases after three to six weeks [4], so it is only prescribed for short-term use. During longer dosing regimens, the side effects of phentermine outweigh the positives.

Phentermine Headaches

Phentermine can cause headaches as a common side effect, precisely the 2nd most common side effect of anti-obesity drugs [5]. Between 2013 and 2020, about 800 reports of headaches after pure phentermine were reported (from a total of 18,675 adverse events reported). This number was lower when phentermine was combined with topiramate at about 400 reports. This is because topiramate may have anti-migraine properties [6].

The reason why phentermine causes headaches is diverse. It can be because of its effects on blood vessel dilation, which could press on the nerves in the skull, causing pain [7]. In sporadic cases, phentermine has been associated with severe headaches. However, these were because of subarachnoid haemorrhage, a type of stroke caused by bleeding in the subarachnoid space surrounding the brain. This highlights the importance of adhering to doses and drug indications when taking phentermine and bringing up all unusual symptoms to medical professionals [8]. The doctors concluded the stroke was a result of hypertension, which phentermine exacerbated.

The prevalence of headaches was comparable to nausea and vomiting side effects [5].

Managing Phentermine Headaches

Phentermine headaches are typically mild and can be easily managed. Some home remedies to relieve these headaches include drinking more water, getting plenty of fresh air, gently massaging your temples, and getting rest and relaxation [9].

It is safe to take over-the-counter analgesic drugs to manage phentermine headaches. These include tylenol, ibuprofen or paracetamol. Try to avoid taking pain medications that also contain caffeine, as both phentermine and caffeine are stimulants. Their compound effect could increase the risk of withdrawal effects, increase heart rate and make it difficult to fall asleep [9].

Other weight loss drugs can also cause headaches [5].

Phentermine Side Effects

Phentermine, like other weight loss drugs, has a variety of side effects, which can range from mild to severe.

  • Constipation: This can be mild to moderate and managed by drinking more water, exercising regularly, and eating fibre-rich foods. If you find this insufficient, you may consider laxatives, but discuss these with your healthcare provider [9].
  • Nausea
  • Cardiovascular side effects include heart palpitations, chest pain, elevated heart rate by up to 20bpm and increased blood pressure [5]. 8].
  • Allergic reactions. such as a skin rash, itchiness, swelling, and trouble breathing [9].
  • Restlessness
  • Feeling intense excitement or happiness or feeling uneasy
  • Trouble sleeping, nausea, dizziness, tremors, diarrhoea, dry mouth and tongue
  • Bad taste in your mouth [9].
  • Erectile dysfunction in men [1].
  • Psychological effects: irritability, personality changes, and in rare cases, psychosis [10].

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Klara Hatinova

Klara Hatinova

Klara is a postgraduate researcher in experimental psychology at the University of Oxford. She has worked across a spectrum of hot topics in neuroscience, including her current project measuring reinforcement learning strategies in Parkinson’s disease. Previously, she studied the efficacy of psilocybin as a therapy for critical mental health conditions and examined molecular circadian rhythms of migraine disorders. She completed her undergraduate degree in Neuroscience at the University of Glasgow and participated in a year abroad at the University of California, where she worked on a clinical trial for spinal cord injury.