Hi Dear user ! I have gone through your questions  and i understand your concern! 
One of the most serious and most often side  of this medication  is infection. This drug block an enzyme that play a major role in the immunity system in the end your immunity system will function less. It's a choice the we make when 
methotrexate is not successful. 
In this condition viruses , bacteria , fungus or others ,take advantage and cause infection ( pneumonia, 
urinary tract infections, Herpes reactivation if you have the virus in your body, cellulitis ...)  be aware of these symptoms :
- cough, 
sore throat, sneezing,stuffy or runny nose,, fever, chest pain,  difficult breathing, burning ,difficult or painful urination, or any other sign of infection.
Allergic reaction: difficult breathing , , rapid breathing .rash 
 Liver problems : Elevated liver test ( ALT/AST)., dark urine or stools , itching, yellow  skin or eyes.
Gastrointestinal problems: diarrhea,  ,nause,vomiting , 
abdominal pain , ..
Hypertension,  
dehydration,  headache, dizziness,   sleeping problem,  muscles  pain...anemia
If you have any of the symptoms  you must go to your local doctor( he /she will stop the medication )
 I would suggest to blood test every 3 month to check your liver, white blood cells , red blood cells,  
cholesterol. 
Wish these help.
If you have any other questions  please feel free to ask me .