'The two ingredients are dextromethorphan - often listed on labels as 'DM' - and diphenhydramine, an antihistamine.
The former is the most common non-prescription cough suppressant available over the counter in the United States.'
When two persons, things or groups have just been mentioned, you can refer to the first of them as 'the former'. In the above example, 'the former' refers to 'dextromethorphan', not 'diphenhydramine'.
The opposite of 'the former' is 'the latter'. It refers to the second of the two persons, things or groups that have just been mentioned (i.e. 'diphenhydramine' in the above example).
Bear in mind that 'former' and 'latter' are used only when there are two persons or things. Below is a very common mistake that should be avoided: 'For dessert, I usually have ice cream, apple pie or chocolate cake. Among the three, the latter [chocolate cake] is my favourite.' The correct word to use is 'last', not 'latter'.