I received an extension on my visa of 60 days due to medical treatment and a doctors certificate saying that it was unsafe for me to travel.

I'm still undergoing medical treatment, can I get another extension (if I provide a new certificate of course)?

With doctors certificate confirming treatment is still a barrier to travelling you should be able to do so. Here is the official rule which deals with this:

Police Order 777/2551

2.25 In the case of medical treatment or convalescence or to look after a patient:

Permission will be granted for a period of not more than 90 days at a time.

(1) Confirmation and request has been made by a physician responsible for the treatment. The Physician must describe the details of treatment and advised that the illness is a barrier for travelling.

(2) In the case of looking after a patient, confirmation and request has been made by the physician responsible for the treatment or by an embassy or consulate.

(3) Patient Carer, except parents, spouse, children, adopted children or children of spouse, shall be granted no more than 1 extra person.

Thanks.

Yes.

Immigration can refer you to their own Dr if they have any doubt regarding your claim and their word is final.