A truth serum is a drug which makes people unable to lie.
Prisoners Lister and Rimmer once spiked Warden Ackerman of The Tank with such a drug, putting into Ackerman's asthma inhaler. When Ackerman later appeared on the bridge of Red Dwarf, he apologised for being late, telling the officers that he had been "having jiggy jiggy with the science officer's wife and hadn't allowed enough time to change out of his Batman costume."
- Like Boing and the programmable virus, the truth serum was presumably stolen and then given to Lister and Rimmer by Bob the Skutter, likely from the Medi-Bay.
- Lister mentions that the truth serum is the real world Sodium Pentothal. However, in the real world, the use of Sodium Pentothal as an effective truth serum has been widely deemed to be false.