@robin47- Welcome to Mayo Connect. I am not sure why your post was not answered in December. I just received it. I apologize for this oversight.
I'm so sorry that your dad is going through this. It must be very very difficult to feel so helpless. Are you his main caretaker? I am a lung cancer survivor but I never had immunotherapy. What type of lung cancer does he have? Small cell (SCLC) or non-small cell (NSCLC)?
I think that you can google Opdivo and see the side effects and how to avoid some of them, if your dad experiences them.
https://www.opdivo.com/
There is some research that indicates that CBD oil interferes with immunotherapy. And as you know, the use of CBD oil and how it interacts with other drugs is far from giving us definitive answers.
- https://www.oncologynurseadvisor.com/general-oncology/cannabis-reduces-immunotherapy-response/article/688059/
- https://oncologypro.esmo.org/Meeting-Resources/ESMO-2017-Congress/The-effect-of-cannabis-use-on-tumor-response-to-Nivolumab-in-patients-with-advanced-malignancies
- https://www.medicaljane.com/2014/08/28/study-cannabinoids-may-increase-lung-cancers-susceptibilty-to-specialized-killer-cells/

The ultimate decision will be up to your dad. Have you asked him about this? What does he want to do? As a cancer survivor of 21 years (NSCLC) I can tell you that unless a cancer patient can not answer questions all decisions should be made by the cancer patient. It is his/her power. I'm sure that you know this.
Please get back to me and let me know if what your dad's decision is. My best, Merry