Paracetamol is the name of a substance, and is normally non-countable. As a non-count noun it doesn't use a singular article, or have a plural form.
It does have a countable sense, meaning "a single tablet containing paracetamol" In this sense "a paracetamol" is correct.
I had a headache so I took a paracetamol and lay down. (one tablet)
If you take paracetamol you should not drink alcohol. (an amount)
How much paracetamol did you take? (amount) How many paracetamols did you take? (number of tablets)
This is the same as "I drink coffee" and "I drank a coffee", where the latter means "one cup of coffee".
You would use a definite article, when the question "Which paracetamol?" can be answered:
I had some asprin and paracetamol in my cupboard, but I threw the paracetamol away, because it was out of date. (uncountable but with a definite article in the second use because it has been definitely identified by the first use)