The agency secured the contract after a competitive pitch. Its PR campaign will help support the brand's ongoing £3.5m marketing campaign, including TV ads, which aim to boost consumer awareness of the brand and de-stigmatise the condition.
The agency will target opinion formers, commission research, handle press office work and stage media briefings.
Biss Lancaster will also promote the website www. constipationadvice.co.uk.
RB has formerly used agencies including Nexus Communications to promote its constipation brands in the OTC market, but took the work in-house last year.