Am I able to sue my co-tenant for rent if they are a co-signer on the lease?

If you have co-signed a lease, you and the other tenant are jointly and severally responsible for paying the rent on that lease. In other words, if the person who co-signed with you does not pay his or her fair share, the landlord will require you to pay it. If you don't, the landlord can sue you and your credit is likely to be damaged. Since your co-tenant is equally responsible, however, you may be able to sue your co-tenant if you pay more than your fair share. The landlord may also sue your co-tenant. 

The key to you being able to sue your co-tenant is to prove that you actually were damaged by her breaching the contract. So, if the landlord comes after you and you end up paying everything, then you have been damaged because you put out a greater amount of money than you should have had to do or than you expected to do. You can then turn around and sue the co-tenant for the amount you spent that he or she was responsible for. Ideally, your lease will specify somewhere exactly how much each of you was responsible for. If it just specified that you both were responsible for the whole, the court will have to try to sort out what your specific agreement was. 

If you intend to sue your co-tenant, you will want legal help that is familiar with the laws in the state you live in. For example, if you live in California, you will want a lawyer in california for sue co-signer tenant.