A drug called 'D-penicillamine' was prescribed to patients in the 1970's to treat arthritis.

An unintended side effect was severe growth of the breasts. I'm not talking a cup size or two, more like breast gigantism.

You can find the full document here:

https://www.ncbi.nlm.nih.gov/pmc/art...0643-0028b.pdf

Here's a few excerpts:

In January 1976 a 41-year-old nulliparous woman who suffered from
rheumatoid arthritis began taking D-penicillamine 750 mg daily. In July
1977 she began to complain of breast enlargement. In June 1978 she discontinued the D-penicillamine for three weeks. There was no reduction in
the size of her breasts and she had severe swelling of both knees. The D-
penicillamine was restarted. In October 1978, after a further slight increase
in the size of her breasts, the D-penicillamine was discontinued. Treatment
with indomethacin was started and in January 1979 was changed to naproxen
250 mg three times a day. In February 1979, five months after the last
exposure to D-penicillamine, the patient was admitted to hospital with severe breast enlargement. She refused surgery.
And furthermore:

How D-penicillamine induces breast enlargement is ill understood. An effect on sex hormone binding globulin might increase the amount
of circulating free oestrogen.' Had D-penicillamine interfered with oestrogen binding the effects should have been apparent in other
oestrogen-dependent tissues. The patient did not show changes in menstrual function while receiving D-penicillamine, nor during the time of maximal breast growth. It is more likely that D-penicillamine produced a local effect on the breast.
I don't know if they've explored further into how this drug works, but if they can turn it into a safe pill treatment then it will be sought after by at least a billion of the 2.5 billion adult women in the world.