Proviron is also not a c17 alpha alkylated compound, an alteration commonly used with oral anabolic/androgenic steroids. Not using this structure in the case of Proviron removes the notable risk of liver toxicity we normally associate with oral drugs. It is therefore considered a "safe" oral, the user having no need to worry about serious complications with use. This steroid in fact utilizes the same 1-methylation we see present on Primobolan (methenolone), another well tolerated orally active compound.
With the structural (c17-AA) alteration, the tablets will also place a higher level of stress on the liver than the injectable (which avoids the "first pass"). During longer or higher dosed cycles, liver values should therefore be watched closely through regular blood work. Although less common, there is still a possibility of liver damage occuring with the injectable form. While it does not enter the body through the liver, it is still broken down by it, providing a lower (but more continuous) level of stress. Such stress would of course be increased with the addition of other c17-AA oral compounds to a cycle of Winstrol. When using such combinations, cautious users would make every effort to limit the length of the cycle (preferably 6 to 8 weeks) and take some form of liver protectants. It should also be noted that both versions of Winstrol have been linked to strong adverse changes in HDL/LDL cholesterol levels. This side effect is common with anabolic steroid therapy, and obviously can become a health concern as the dose/duration of intake increase above normal. The oral version should have a greater impact on cholesterol values than the injectable due to the method of administration, and may therefore be the worse choice of the two for those concerned and this side effect.
So it looks like Winstrol is more liver toxic. And oral winstrol is more liver toxic the injectable. I assume that drinking an injectable would be the same as oral winstrol and therefore be as toxic. However if cholesterol is a concern it looks like injectable is worse then oral for that.