In many drug plants, the active constituent is present in the amounts of 1 or 2 percent by dry weight of the plant, so when you give an herbal preparation, you're giving something very dilute, and the rise of concentration of the drug in the bloodstream and in the target organs is very slow. Therefore, the effects are much muted, and the toxicity is also much less. Whenever you concentrate therapeutic effect, you inevitably concentrate toxicity. If what you get for your money is death from a pharmaceutical drug, that's not a very good bargain. Modern, science-based medicine is the way of the future.
Alternative medicine belongs to the past.
It is old wine, albeit sometimes now being served in new bottles, and is unlikely to produce new knowledge or improve future medical care. It has a strong appeal to those who want to believe in a human-friendly universe, and in the benevolent healing powers of nature. Alternative medicine can be a compelling hobby for the healthy, and a final hope for the dying. But however valuable these things may be for some of us, we ought not to forget that our best chance for a healthier future rests on continued advances in conventional medical practice, based on science and objective evidence. It's the patient that suffers in this.