I had laparoscopic surgery 2.5mths ago for ectopic pregnant followed by shot of mtx. can mtx cause weak immunity or viral infections due to low immunity?

Yes: Methotrexate does lower your immunity temporarily--if you are concerned about infection, you should speak to your doctor.

No: A single dose of Methotrexate has little effect on immunity.

Not likely: It's unlikely that one dose of Methotrexate will lead to a significant change in your immune response to cause an infection. More importantly, if you had a laparoscopic procedure to remove the ectopic pregnancy, you shouldn't have needed the Methotrexate shot which is reserved for patients that opt for medical treatment instead of surgery.