Welcome to HW -
Before I realized that this post was on the current thread about tetracycline, I was off to find it to link to.
I've not seen any other material on this, but I would suspect it would not be difficult to get a prescryption for either of the two.
From my experience as a patient only - There is a risk of either kidney or liver damage (I don't remember which), and that leads me to have to have scheduled blood tests every six months, as I have been on one or the other for years. For the fair-skinned, you also have to stay out of the sun. Hat, long sleeve shirt, and sunglasses at all times when outside. At higher dosages, you get a higher probability of fungal infections, and bowel problems.
The question would be dosages. I have to think that dosages of a size to impact cancer progression would be high - raising the risks. But then the risks may not be significant in comparison.
I've taken both as 250mg capsules, and 100mg tablets (in a form that was intended for dental infections - when you need it, you take what they will give you).
Both of the two are commonly available and very inexpensive as generics in the US. I've also found both in Europe in some form when needed.