How treatment is given Back to top. You'll be given folic acid and vitamin B12 to take before your treatment with pemetrexed begins and throughout your chemotherapy. It's very important to take folic acid and B12, as they reduce the side effects of pemetrexed.Folic acid can be taken as a tablet.ow treatment is given Back to top. You'll be given folic acid and vitamin B12 to take before your treatment with pemetrexed begins and throughout your chemotherapy. It's very important to take folic acid and B12, as they reduce the side effects of pemetrexed.