My dear Remus,
I've reviewed the potions and I can see your concern. The first is what I would consider a very rough and ready sort of brew, a bludger instead of a snitch and lacking in subtlety. Reviewing the ingredients, however, and the process, I believe the warnings are in good faith and overall the potion is as safe as anything which contains such a large quantity of white arsenic. The potion base is a common variant of the Lorcas buffer, and creates a time delay effect. This both mitigates the toxicity of the overall potion while extending its full effectiveness from several minutes to several hours. This variant is a six hour version.
The second recipe is a stripped down variant of an extremely common purgative popular about 30 years ago. There's nothing inherently wrong with it, merely certain ingredients to improve the taste and its holding ability have been removed in an effort, it appears, to make the brewing simpler and to make the potion slightly more effective. There have been advancements in the area since, of course, but I hesitate to recommend an alternative as I am unsure regarding the potions interaction given your particular case.
Looking at the two and considering they are to be taken in tandem, my professional assessment is that the potions are safe enough for a person of generally sound constitution, without lycanthropy. The side effects listed are all well within the predictable range, and are consistent with moderate poisoning. That said, I doubt this 'cure' will be pleasant in the least. For a person without lycanthropy, my educated opinion is that this potion would do nothing, except result in a few very unpleasant days of cramps, intestinal distress, etc.. I regret to say I am insufficiently conversant with the effects of lycanthropy to predict whether the potion will effect a cure, however. All I can say is that I do not believe the potion would do you
lasting harm if prepared and taken in the way stated, and followed by the purgative in good time. The final decision is, of course, yours. I do recommend that you have Poppy on call if you choose to go through with this purported cure, as she has a great deal more expertise dealing with illness and poisoning than the rest of us.
Do let me know if you have any other questions, lad. I'm glad to help and anything which might return your magic to you is of great concern to me.
Your friend,
Horace Slughorn