If you have mono, it really probably is not a good idea to drink alcohol. If you do it could make you feel extremely unwell.
With mono, as well as feeling really lousy, you may have enlarged lymph nodes in your neck, throat, axilla, messenteric nodes in your belly, and sometimes your liver and spleen can become enlarged. This is why after diagnosis, your doc may tell you not to take part in contact sports. If your spleen is enlarged, it is more liable to be ruptured. Your spleen becomes enlarged, as it is one of the largest lymph organs in the body, although it is possible for you to live without it.
Just FYI, I can't drink hardly at all since being diagnosed with mono. My doctor tested liver function and all is normal, but if I drink alchohol (more than 3 mixed drinks or beer) I feel HORRIBLE. My doctor said that it's normal when you have mono and that I shouldn't drink.