Rabbi Yehoshua ben Levi raised a dilemma: If the High Priest scooped and died, what is the with regard to the possibility that another High Priest may replace him and enter with his handful? May the second priest enter the Holy of Holies with the incense that the first priest scooped, or must he start from the beginning of the process? Rabbi Hanina said to his students in excitement: Come and see that Sages from a later generation were able to ask a difficult question on par with the question of the earlier generations. Even I, Rabbi Hanina, asked this same question, which was posed by my elder, Rabbi Yehoshua ben Levi.
One of the proofs brought by the Gemara that Rabbi Hanina was the more senior of the two is a statement made by Rabbi Yehoshua ben Levi that he had received a ruling from Rabbi Hanina allowing him to drink shahalayim on. The fact that Rabbi Yehoshua ben Levi turned to Rabbi Hanina for direction in this matter is understood by the Gemara to mean that Rabbi Hanina was the older man.
Shahalayim is identified as juice of Lepidium sativum L., or cress – an annual herb commonly known as peppergrasses or pepperwort. It is generally used as a spice or salad green. Its fruits can be used as a medicine when ground up and mixed together with wine or vinegar, which was common practice in the time of the Talmud.
The issue of drinking shahalayim on stems from the Rabbinic prohibition against taking medicine on. This includes not only food and drink that is taken for the purpose of healing, but also activities that are done solely for reasons of health, such as exercise, washing, etc. This, of course, applies only to situations where all that is involved is an issue of pain or discomfort. If there is a danger, or just the possibility of actual danger, then even activities forbidden by the Torah would be permitted, since piku’ah nefesh – danger to life – overrides the laws of.
The basis for the Rabbinic injunction is the concern lest someone were to grind and prepare herbs for use as medicine, which involves transgressing the melakhah of tohen on. Once the Sages applied this rule, they employed it across the board, even in cases that do not involve grinding.