Ascendis Animal Health applied in 2011 for revocation of Merck’s South African Patent 1998/10975 under section 61(1) of the Patents Act 57 of 1978, alleging lack of novelty and lack of inventive step (obviousness). While revocation proceedings were pending, Merck instituted patent infringement proceedings against Ascendis. The parties agreed to stay the infringement action pending finalisation of revocation proceedings. The Commissioner of Patents revoked the patent solely on the ground of lack of novelty, without deciding obviousness. On appeal, the Supreme Court of Appeal overturned that decision, upheld the patent’s novelty, and certified all claims as valid under section 74 of the Act. Thereafter, in the infringement action, Ascendis sought to amend its plea to abandon novelty, retain obviousness, and introduce a new defence of inutility. Merck opposed the amendment and pleaded res judicata. The High Court (as Commissioner of Patents) refused Ascendis’s amendment, holding that the validity of the patent was res judicata following the SCA decision. Ascendis unsuccessfully sought leave to appeal in the SCA and then approached the Constitutional Court.