It's only ever been shown to cause tumors in rats. Without looking at the specific studies, I can't tell you what the relative potency in rats was or the dose per kilogram used. Often, in those studies, the doses are much higher by weight than what is given for therapeutic use in humans, because they're attempting to predict worst case long term side effects in an unrealistically short period of time. Considering Spiro was developed in the fifties, and all the literature I've read explicitly stresses it can cause tumors in rats with no mention of human test results, it's not something I'd be overly concerned about. The day I grow a tail and start having to trim my front teeth, I'll start panicking.