When McManus comes into Toby and Chris' pod, a guard starts to walk into frame on the upper level. In the following shots of McManus, the guard is walking the other way with an inmate and then there's a lone inmate, the guard and the inmate and then the lone inmate ties his shoe twice.
Prison inmates, especially at a maximum security facility, would not be allowed to handle and use rat poison unsupervised. In fact it is highly unlikely they would be allowed to use it even under direct supervision.