Panadol have another name called Paracetamol or Acetaminophen.
Panadol may have side effect damage your kidney and liver but it is more important not to overdose and drink plenty of water; take Panadol with cold water is more desirable (My doctor said).
The following information I found from a web page hope it may help you.
http://www.3dchem.com/moremolecules.asp?ID=9&othername=Panadol#
"Acetaminophen is completely absorbed from the gastrointestinal tract and, after intake, peak plasma concentrations are reached in less than an hour. The drug is fairly uniformly distributed in the body and approximately 90% of a therapeutic dose is eliminated by conjugation with glucoronic acid in the liver; 3-5% is catabolized to the acid and cysteine conjugates by the P-450 mixed function oxidize enzyme system. All of these metabolites are excreted in the urine and in fact only a slight amount of the drug is excreted unchanged. It is the intermediate metabolites formed during the biotransformation in the liver (whose structures are uncertain) that are believed to be responsible for the hepatotoxicity of the drug."
Please don't worry about too much, but make sure not to overdose Panadol. If you persist have any unbearable pain during your menstruation, it is better for you to see doctor.