"Can they be taken at the same time - before bed?"
Tamoxifen is a chemotherapy agent commonly used in the treatment of breast cancer. There is a research gap concerning the anti-tumor effects and effectiveness of cannabis or cannabinoids in treating cancer generally. There is therefore insufficient evidence to support or refute the conclusion that cannabinoids are an effective treatment for cancers.
However, there is conclusive evidence that oral cannabinoids are effective antiemetic in the treatment of chemotherapy-induced nausea and vomiting (Committee on the Health Effect of Marijuana: An Evidence Review and Research Agenda). Cannabis is also well known to be an effective adjuvant analgesic for pain treatment.
Cannabis contains THC and CBD but also compose of over 100 cannabinoids, about 400 terpenes and flavonoids. The interaction of cannabis with most other drugs is not well known. Whatever the reason for which you are consuming cannabis, as a safety measure, I recommend that clients take other drugs and supplements at least two hours apart from cannabis products.
Drug-drug interactions are a potential concern if you are taking more than one medicine at a time. Cannabis and its main cannabinoids, THC, and CBD, may cause drug interactions. Tamoxifen is a “pro-drug,” which means it is not fully effective until it is metabolized by the liver into its more potent anticancer form through a specific metabolic pathway in the liver.
This metabolic process can be interrupted, inhibited, or even entirely blocked by other substances, includinf Cannabis. Therefore , I would recommend separating these two medications for at least two hours in between.
As others have mentioned, the research regarding drug interactions with cannabis is very limited at this time. As tamoxifen is an antiestrogen that is heavily metabolized in the liver, the potential of an interaction exists.
Cannabis is known to affect liver metabolism via the p450 enzymes, in some cases increasing serum levels of other medications, and sometimes decreasing those levels. Unfortunately, we just don't have the research at this point to provide a definitive answer. As always, start low and go slow.