Valmiki Ramayana Sundara Kanda Sarga 1 Shloka 21
Original Shlokaयानि चौषधजालानि तस्मिन् जातानि पर्वते। विषघ्नान्यपि नागानां न शेकुः शमितुं विषम्।।5.1.21।।
1 source checked for this shloka
तस्मिन् पर्वते on that mountain, जातानि grown, यानि those, औषथजालानि medicinal herbs, विषघ्नान्यपि even those which can counteract poison, नागानाम् of the snakes, विषम् poison, शमितुम् to neutralize, न शेकुः were not able to.
Even the medicinal herbs grown on the mountain could not neutralize the effect of the poison of snakes.