Fajr Azan is in progress I remembered my daily permanent medication for HBP-which usually I get soon after Sohoor meal. Doubtfully I swallowed the tablet before Azan was over and continued
My fasting-----what would be the rule here?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

If the Athaan is called (the call indicating the time of prayer) just at the appearance of the dawn twilight, which is usually clear in the horizon, then one should stop eating and drinking right at the start of the Athaan regardless of whether what is being eaten or drunk is medicine, etc. Allaah Says (which means): {… and eat and drink until the white thread (light) of dawn appears to you distinct from the black thread (darkness of night), …} [2: 187].

So, whoever eats, drinks or does anything else that usually nullifies the fast, after the Fajr (dawn twilight) appearance then his fast becomes invalid.

An-Nawawi said: 'We mentioned that when the Fajr appears while one has something in his month, he should spit it out and complete his fasting. But if he swallows it then his fasting is nullified and there is no disagreement over this issue. The evidence for this is the narration of Ibn 'Umar and 'Aa'ishah, that the Prophet said: "Bilaal calls Athaan before dawn. So, eat and drink until Ibn Um Maktoom calls Athaan" [Al-Bukhari and Muslim]'.

As for the narration that is traced back to Abu Hurayrah and that says: "If the Athaan is called and one of you has a receptacle in his hand, he should not leave it until satisfying his needs". Al-Bayhaqi said this narration is according to the majority of Muslim scholars, linked in its meaning with the fact that the prophet knew that Athaan is called before the Fajr.

To conclude if as mentioned in your question, Athaan is called right at dawn then your fasting was nullified and you should make up for this day.

Allaah knows best.