Woman taking Mounjaro weight loss jab reveals unexpected side effect as breasts grew to 34NN

When Tianna Moon lost three stone on Mounjaro, it seemed like everything was going right. But while she wanted her waist to shrink, she never expected the weight loss medication to make her breasts bigger.

The 30-year-old had 'always' had a larger chest, but after deciding in May 2024 that she wanted to lose some weight, her breast size ballooned from an L cup to an M. This means that her bust's circumference grew by around an inch.

While the content creator from the UK thought it was 'odd' that her breasts were growing while her weight was shrinking, it wasn't until she looked at online health forums and researched using ChatGPT that she realized something was wrong.

Now diagnosed with gigantomastia, a rare condition that causes a rapid and extreme growth in breast tissue, Moon has seen her chest size increase further and now struggles to find any bras that can support her.

The 30-year-old content creator went from an L cup to a 'NN' (SWNS)

But Moon, who suffers serious back pain and sores as a result of her condition, says she has found ways to 'make it work', buying the biggest size of bra possible and squeezing into it.

However, this has caused painful cuts and tears under her breasts, which grew to 34NN as she lost a further two stone. She explained: “Bras are impossible. Bras in my size NN don’t exist.

“I decided to lose weight and eventually started on Mounjaro and lost three stone over the course of a year. In that time my breasts grew from an L cup to an M cup."

She has since 'stepped away' from bras entirely as a result of the pain they cause, deciding that she is 'happier without them'. But more than half a year after her July 2025 diagnosis, she is still struggling with her expanded bust.

“The side effects are like having numb arms quite a lot and having quite severe indentations on my shoulders," Moon shared. "I have some light scarring under my breasts where bras have rubbed and ripped the skin open.

“Also laying on my back is not something I can do for a long period of time as it restricts the amount of air flow I can get in”.

After suffering a head injury in 2021 and becoming unemployed, Moon decided to join OnlyFans to earn some money from the extra attention that her then-size L chest got from strangers.

Moon said: “It got to the point where last September I was in Amsterdam for the Xbiz awards as I was nominated. I was walking around with another lady and she said, ‘don’t you realise those people are pointing at you and trying to get your attention?’

“It’s become so normal to me I don’t notice it anymore”.

Tianna is exploring the possibility of breast reduction surgery (SWNS)

But when she dropped from 19st to 14st and her chest size went up by three cup sizes, Moon decided to look into what could be causing this severe change. Having heard about a less drastic breast tissue growth condition, she decided to speak to her doctor.

She said: “Originally, he was looking at macromastia, but when I broke down medical history, the weight loss on Mounjaro and the increased growth and side effects, he was point-blank like ‘this is gigantomastia’.

“I got the diagnosis quite early because of the growth not being common with anything else. At that point, I decided to get rid of the extra weight and protect my back because of the diagnosis."

Her breasts now collectively weigh around three stone, causing her daily pain. In November, Moon took further preventative steps to protect her health, paying to get a gastric sleeve fitted to prevent even more extreme back pain in the future.

But while she is considering breast reduction surgery, the OnlyFans creator has been raking in the cash since developing gigantomastia, earning around $13,000 per month.

She said: “I might as well try and reap the benefits of my medical condition rather than the negatives."

And it was actually one of her fans who first noticed that her chest was expanding. She added: “As they got bigger, it was actually a subscriber who said you need to get remeasured because you are not the size say you are."