The problem with drugs is that many have side effects or can interfere with other medications. In a cycling incident scenario this can be an issue
User13710's post, whilst intended to discredit, is actually a superb support of the responsibilities and liabilities of giving drugs to strangers
You sound like those people who used to say no one should attempt to do CPR in case they did more harm than good, like breaking the poor dying person's ribs. .
If you perform CPR and do break ribs then as you are performing an approved First Aid process and in the approved manner then you are covered and there will be no come back.
However if someone reacts to a drug or medicine that you have given then as it is NOT an approved First Aid process, you are open to claims in the worst case
The difference is subtle, but important
If someone says to you "Do you have any Gaviscon" and you provide the two tablets then it is their request and their decision
However if someone says that they have "heartburn" and you then suggest that they take Gaviscon and they react it is your responsibility as you ahve technically decicded what is wrong with them and given medication
Not forgetting that you have decided it is Heartburn and Gaviscon is appropriate. Heartburn can be the first early warning of a heart attack, which you have excluded?