In clinical studies, gaining weight while taking Singulair was not reported as a possible side effect. There are a number of possible side effects with Singulair ® (montelukast sodium). Weight gain, however, does not appear to be one of them.This conclusion is based on the results of clinical trials, where Singulair was extensively studied and side effects were documented.or people taking Singulair, weight gain was not reported as either a common or rare side effect. If you are noticing an unexplained increase in weight while taking Singulair, there are some things that you can do.