For example:  digoxin (digitalis), from foxglove.  For congestive heart failure, though less popular than it used to be.  A couple of leaves of foxglove can kill you.

taxol, from the yew tree,  chemotherapy.  inhibits cell division.  now synthesized, primarily because the original source, the Pacific yew) was decimated years before the discovery of taxol as a "trash tree."

vincristine-from periwinkle.  another chemotherapeutic agent.

The prejudice against marijuana has been political, not scientific.  There are many oncologists who have found that when their patients smoke pot, they have much less pain and nausea (a side effect of much chemotherapy is that it strips the lining of the stomach and intestines).  In smokable form, the patient typically titrates the dose (uses enough to get relief from the problem).  Injecting THC (one of the more active components of marijuana) does not give the same degree of relief and has more side effects.