I am a pharmacist who previously owned, and now work as a locum. I was a long term Sigma supporter.
In my travels I have worked for a couple of these majors and can confirm that they are indeed getting at least 6 months credit and special pricing. They don't even support Sigma generics, despite all other groups like Guardian or Amcal needing to hit certain guidelines about stocking Sigma products and support to get their much smaller extra discount.
I guess it must have been tempting to offer Chemist Warehouse good terms when then were smaller but it seems to me to be a great example of "if you owe the bank $100K and can't pay you have a problem, if you owe the bank $100M and can't pay, they have a problem".
My guess is that most Chemist Warehouse would be turning $5-10M pa so purchases from Sigma of $3-6M pa. So six months credit for $2.25M (purchases in 6 months) times 150 stores is a rather large number ($337.5M).
If Sigma changed their terms and they couldn't pay, major problem.
CW seem to be financing their growth from buying a rundown pharmacy for $2M, fitting it out, filling it with stock and suddenly Sigma's terms have "paid" for that purchase, until some time in the future when the pharmacy is sold. A bit like putting it on your Mastercard.
Sigma has had problems since they paid too much for Arrow Pharmaceuticals and I guess they were getting desperate.