Low-dose methotrexate is a common treatment for several types of arthritis (including rheumatoid arthritis) and other inflammatory conditions. High-dose methotrexate may be given to people with certain types of cancer as a part of their chemotherapy.

The recommendation for men to wait up to 6 months after stopping the drug to conceive comes from the combination of the lifespan of sperm (up to 90 days) and how long methotrexate stays in the body after it is discontinued (up to several months in the liver).

There are at least two reasons that doctors advise men to stop taking methotrexate prior to fathering a child:

1. The drug may lower the sperm count and impair fertility.
2. Methotrexate could cause mutations or chromosomal damage to sperm. Although this damage is well-established in mice, it's not clear whether it occurs in humans.

Despite these concerns, the safety of methotrexate is quite good for men who are considering fatherhood. That is, most men who have taken methotrexate in the past are able to father normal, healthy babies.

However, the stakes are higher for women. Taking methotrexate during pregnancy can cause terrible birth defects. So, there are strong recommendations to avoid taking this drug for women contemplating pregnancy.