Many women take phentermine to increase fertility or experience unplanned pregnancies shortly after taking this medication. That combination of circumstances leads a lot of people to wonder: does phentermine increase fertility? The short answer is [often] yes… but not because the pill has any special powers. In both men and women, carrying extra weight decreases the chances of successful conception, so weight loss (by any means, including with phentermine) in one or both partners can increase fertility. Still, the FDA considers this medication category X, which means that phentermine should NOT be taken by women who are trying to conceive, pregnant or lactating. For that reason, it’s important to use reliable birth control while actively undergoing treatment. Below we’ll talk more about the complex relationship between weight loss, phentermine and fertility.
Weight Loss and Fertility
Most people know that successful conception and a healthy pregnancy involves a careful balance of hormones, but did you know that body weight – and more specifically body fat – can have a major effect on this natural equilibrium? This relationship between body weight and hormone levels helps explain the undeniable link between weight loss and fertility.
In fact, one study showed that:
“The probability of a spontaneous pregnancy declined linearly with a body mass index (BMI) over 29 kg/m2. Corrected for possible related factors, women with a high BMI [over 29 kg/m2] had a 4% lower pregnancy rate per kg/m2 increase.” 
That means that, for example, the average 5’5″, 193-pound woman (starting BMI of 32) who loses just 10 pounds could boost her probability of getting pregnant within a year from 84% all the way up to 92%!
Hormonal Imbalances and Infertility
Hormonal imbalance is a major cause of female infertility in overweight and obese women who have a BMI ≥25 kg/m2 or ≥30 kg/m2, respectively. These potential moms are at higher risk for hormonal imbalances due to:
- Excess body fat
- Insulin resistance
Weight loss and fertility are linked because losing even a small about of weight before trying to conceive can make a big difference in the journey to re-establish this balance.
Excess Adipose Tissue
Estrogen levels, in particular, vary with weight gain and loss because this hormone is produced not only in a woman’s ovaries, but also in her adipose tissue (fat). Due to the increased manufacturing space, women that carry extra body fat may experience higher levels of estrogen. This can interfere with fertility because high levels of estrogen cannot be countered by normal levels of progesterone and testosterone, which throws off the body’s carefully-calculated hormonal balance. As a result, high levels of estrogen can inhibit proper ovulation.
It’s worth noting that being significantly underweight or over-exercising can also negatively impact hormones in a way that reduces or prevents ovulation.
Insulin resistance is another problem that can impact female fertility. Pre-existing insulin resistance decreases a woman’s fertility and increases the risk of complications during pregnancy. Unfortunately, overweight and obese women are more likely to suffer from insulin resistance.
Insulin is the hormone responsible for moving glucose – the sugar our body uses for fuel – into cells for energy. When someone has insulin resistance, the body’s cells grow desensitized to the hormone’s cues. As a result, the cells don’t receive the fuel they need and the liver continues to release more and more glucose into the bloodstream. Upon detecting the high levels of glucose, the pancreas pumps out more and more insulin, which the cells continue to ignore.
Unfortunately, in response to these high levels of insulin, the ovaries also produce less estrogen and more testosterone, which can interfere with ovulation.
Insulin resistance may be associated with polycystic ovary syndrome (PCOS), but they two conditions aren’t always related. Some women experience insulin resistance in the absence of PCOS, which can lead to type 2 diabetes. Talk to your gynecologist if you show signs of insulin resistance or PCOS.
Phentermine and Fertility
REMINDER: Phentermine should NEVER be taken while trying to conceive (TTC), pregnant or breastfeeding. Tell your doctor if you are in any of these stages. Phentermine is FDA Category X.
Due to the relationship between weight loss and fertility discussed above, many women find themselves pregnant during or shortly-after treatment with phentermine. There are no studies that show phentermine itself increases woman’s chances of getting pregnant, but there is an undeniable link between weight status and fertility.
So, it’s common for woman who get closer to a healthier BMI with phentermine to experience an increase in fertility. A lot of people – both men and women – also experience a boost in libido while taking this medication. The combination of higher sex drive and increased fertility may also increase the likelihood of a “phentermine baby”.
Phentermine Use (Categorization)
No matter how eager you are to conceive, it’s important to keep in mind that phentermine should NOT be taken by women who are currently:
- Trying to conceive (TTC)
- Lactating / Breastfeeding
Be honest with your doctor if you are trying to conceive, suspect you may be pregnant or still breastfeeding. The two of you can work together to develop a weight management plan that will be best for you and your [potential] baby.
FDA Recommendations (US)
Phentermine is FDA pregnancy category X in the United States. The US Department of Health & Human Services’ CHEMM website defines category X medications as those for which, “studies in animals or humans have demonstrated fetal abnormalities and/or there is positive evidence of human fetal risk based on adverse reaction data from investigational or marketing experience, and the risks involved in use of the drug in pregnant women clearly outweigh potential benefits.” 
There have not been any controlled studies in humans that specifically looked at the effect of phentermine on pregnant women or their offspring. In animal studies, phentermine administered at 7x the maximum human dose (MHD) in late gestation did not cause maternal or fetal harm. However, when administered at 10x the MHD, phentermine stopped estrous cycling (comparable to menstrual cycling in humans). 
Phentermine passes into the breast milk and could be harmful to a baby, so lactating mothers should avoid this medication.
TGA Recommendations (AU)
In Australia, phentermine is TGA pregnancy category B2. According to the TGA website, this category encompasses “drugs which have been taken by only a limited number of pregnant women and women of childbearing age, without an increase in the frequency of malformation or other direct or indirect harmful effects on the human fetus having been observed.” 
Phentermine and Male Fertility
We’ve talked a lot about the effect of weight loss and phentermine on female fertility, but what about the guys?
Turns out weight loss affects male fertility in many of the same ways it affects female fertility. Over the past few decades many studies have linked higher female BMI (and more indirectly, adiposity) with decreased fertility. Emerging research now indicates that the potential father’s BMI matters too.
Obesity and Male Fertility
Obesity decreases men’s fertility because it affects sex hormones, reduces sperm quality and may negatively impact embryonic health. Similar to obese women, obese men also produce more estrogen and less testosterone, which ultimately decreases fertility and sperm count in overweight men.
Due to these effects, couples take longer to conceive and have higher odds of pregnancy loss when the man is overweight obese. Plus, obese fathers are more likely to have an obese child.  So, in cases where the hopeful father is overweight or obese, it’s beneficial for him to lose weight before trying to conceive.
Weight loss can also improve a man’s sex life, which makes getting pregnant easier. Erectile dysfunction is a common problem among obese men, but studies show that losing weight significantly improves sexual function. Plus, weight loss – with phentermine or otherwise – usually increases sex drive.
Weight Loss and Male Fertility
The good news is that simple weight loss interventions like healthy diet and increased activity make a big difference with male fertility. Both sperm count and child outcomes are correlated with the father’s metabolic health, so losing a few pounds and focusing on physical activity can dramatically improve results.
Phentermine and male fertility are related because men (like women) experience better odds of successful conception the closer they are to a healthy weight. Phentermine weight loss pills, along with healthy diet and regular exercise, can help you reach this goal.
Have an opinion on phentermine and fertility? Share your thoughts and experiences with us in the comments section below!
- Steeg, J. W. et. al (2007). Obesity affects spontaneous pregnancy chances in subfertile, ovulatory women. Human Reproduction, 23(2), 324-328. doi:10.1093/humrep/dem371
- US Department of Health and Human Services. (2008, May 29). FDA Pregnancy Categories – CHEMM. Retrieved from https://chemm.nlm.nih.gov/pregnancycategories.htm
- Drugs.com. (2018, July 26). Phentermine Use During Pregnancy. Retrieved from https://www.drugs.com/pregnancy/phentermine.html
- Australian Government Department of Health. (2011, May 03). Australian categorisation system for prescribing medicines in pregnancy. Retrieved from https://www.tga.gov.au/australian-categorisation-system-prescribing-medicines-pregnancy
- Palmer, N. O., Bakos, H. W., Fullston, T., & Lane, M. (2012). Impact of obesity on male fertility, sperm function and molecular composition. Spermatogenesis, 2(4), 253-263. doi:10.4161/spmg.21362