I believe she was poisoned
Here is a wonderful answer from Meg at alwaysmarilynmonroe:
My opinion is very mixed due to the mess of the autopsy however and the fabricated/changed accounts by Dr Ralph Greenson and Eunice Murray. However, after reading Donald Spoto’s wonderful biography about Marilyn I firmly believe her death was caused by an enema, which was probably instated by Eunice Murray or Dr Ralph Greenson. The autopsy states that there was no pill residue within Marilyn’s body so that makes it believe that the dose would only of been possible by lethal injection or an enema, which I believe as Marilyn often used them. However, it also states that there was no bruising on Marilyn’s body so lethal injection is unlikely and as her colonol was so badly bruised it is pretty obvious that the lethal enema killed her. There was also no water running in the house, and as Marilyn was known to gag on pills unless she had water with them, it is unlikely that she took them all at once. Even though Marilyn was known to of taken many suicide attempts, she was always revived so it is questionable. Also the fact the first police man on the scene, Sgt Jack Clemmons strongly believed it was a staged murder scene. Furthermore, the autopsy was done so messily with all organs not being looked at and thrown away so therefore the whole thing is very suspicious and therefore my answer is very mixed. All of witnesses around Marilyn in the last months of her life have said how Marilyn was happy and looking forward to the future, she was becoming more independent and removing negative people out of her life.However, because of the endless conspiracies and that it is unlikely of ever finding out the truth I try to focus on her wonderful life and the person she was and is almost fifty years later.