If I am not mistaken Blue Mass was a medicine which was perscribed for syphyllis. But it and Marijuana in the North were prescribed for almost everything else also. So I am not positive that Lincoln had syphyllis.

Strangely that was one of the advantages of southern medicine during the war was that because they couldn't get "prescription medicines" of the time such as Blue Mass, they used home remedies and homeopathic medicines. The results was less drug addictions and side effects. For example, instead of using Morphine and Opium the southern doctors used a homeopathy drug for pain made from lettuce. While not as effective as Morphine, it did work and didn't have the severe addiction problems.

But that has nothing to do with Lincoln having cancer. But he did appearently have some possibly serious medical and mental problems which may have effected his thinking, including probably what we would call clinical depression.