Many people report few or no side effects, and virtually no one considers them to be "terrible". Now, some find the taste not at all pleasant (which is pretty much remedied if my instructions are followed for taking half under the tongue, mixing the other half in a glass of juice such as pomegranate, waiting at least 30 seconds and then taking a drink of the juice to wash down the oleander under the tongue.). The new capsules, which sell for $70 including shipping are another option.
I think filtering has a lot to do with side effects - and recommend a final filtering through 3-4 layers of unbleached coffee filters. Almost no one who follows that final filtration step ever reports side effects - and it is worth noting that the makers of the patent medicine version, which is filter the oleander extract through less than 4 micron filters report no side effects from the oral use. Relative health may also have something to do with whether any of the normally mild to modest side-effects of nausea, loose stools and minor temperature are experienced. Frequently a person on an effective cancer protocol will feel worse initially as his supplement and body begin to fight the cancer cells and the toxins and lactic acid from the cancer cell are released.
Myself, I never experienced anything except for perhaps a bit of loose stool. That and going blecch until I figured out to do the half and half under the tongue and juice routine.