A 30-year-old woman from the UK has spoken out after experiencing an unusual side effect from the popular weight-loss injection Mounjaro (tirzepatide). Despite significant weight loss, she found that her breast size increased dramatically, an outcome she did not expect when she began treatment to shed pounds.
The woman, identified as Tianna Moon, had been using the weekly injection to lose weight and saw her waist shrink while her breasts continued to grow. Over time, her bust went from a cup size that most people would consider large to an extremely uncommon size — leading to physical discomfort and health issues.
From Weight Loss to Rare Growth Condition
When she started using the drug in May 2024, Tianna had been trying to manage her weight. With the help of Mounjaro, she lost about five stone (around 70 pounds) over the year but noticed something unexpected: rather than shrinking like the rest of her body, her breasts got significantly larger. After discussing the changes with medical professionals and reviewing her history, doctors diagnosed her with gigantomastia, a rare condition that causes excessive growth of breast tissue. This diagnosis came after observing that her body was reacting in an unusual way to the combination of weight loss and possible hormonal shifts. Gigantomastia isn’t directly caused by the drug itself, but in this case, the extreme weight loss and hormonal changes appear to have triggered the condition. This has led to chronic back pain, trouble finding supportive clothing, and even breathing difficulties at times. She has described having toforgo traditional bras because none in the required size are made, which has resulted in sores and discomfort.
Managing Side Effects and Health Challenges
Tianna explained that her breasts now collectively weigh a significant amount and have led to various physical issues. She mentioned numbness in her arms, indentations on her shoulders, and cuts beneath her breasts due to ill-fitting garments. These problems have affected her daily life and mobility. Despite these difficulties, she has adapted by making practical adjustments and has even turned some of the attention her condition brings into a source of income by using platforms where her unique experience attracts interest. Yet she is exploring the possibility of breast reduction surgery to alleviate chronic discomfort and protect her long-term health.
What Experts Say About Mounjaro Side Effects
Health professionals note that Mounjaro, originally developed to help manage type-2 diabetes, also tends to reduce appetite and support weight loss. It works by mimicking hormones that reduce hunger and help regulate blood sugar. However, like all medications, it comes with a range of potential side effects. Most common reactions tend to be gastrointestinal — such as nausea, vomiting, diarrhoea, and constipation — and are often temporary when patients begin treatment. But rarer and more serious effects, such as acute pancreatitis or allergic reactions, are possible and are listed in the drug’s patient information. Recent regulatory updates have added warnings about serious but uncommon conditions such as sudden inflammation of the pancreas, which can cause intense abdominal pain and require urgent medical care. Patients are advised to seek help immediately if they develop symptoms like severe stomach pain with nausea.
Broad Picture: Popularity and Caution
Medications like Mounjaro have surged in popularity as options for weight management alongside lifestyle changes. Some studies suggest they can be more effective than older injectable drugs, leading many to adopt them for significant weight loss. At the same time, growing reports of unusual and serious side effects have prompted health professionals and regulators to emphasise careful monitoring. Individuals considering or using these injections are urged to stay in close contact with their healthcare provider, watch for both common and rare effects, and report any concerning reactions early.
