Multi-drug tablets of amlodipine besylate and atenolol were prepared as either mono-layer (mixed matrix) or bilayer tablets containing each drug in a separate layer by using similar excipients and processing. Each tablet batch was packed in strip and blister packs and kept under accelerated temperature and humidity conditions. The stability of two tablet and packaging types was compared by HPLC analysis after 0, 1, 3 and 4.5 months and expressed as the content of intact amlodipine and atenolol. The content of atenolol did not decline regardless of tablet and packaging type. Amlodipine content in bi-layer tablets decreased to about 95 and 88% when packed in strips and blisters, respectively. When prepared as mono-layer tablets, the content decreased to 72 and 32%, respectively.
The study revealed that the bi-layer tablet formulation was more stable than the mono-layer type. Further, the stability was increased when the tablets were packed in aluminium strips as compared to PVC blisters.