Forging a doctor's signature is indeed a felony in most states.
Forging of a doctor's signature falls under the category as forgery and can get you in serious legal trouble and even jail or prison time.
However the legal consequences of forging a doctor's signature will depend on the jurisdiction and circumstances and whether or not the forged doctors signature is used in order to obtain prescription drugs or used for other fraudulent purposes.
Forgery is a crime and intentionally imitating someone else's signature in order to deceive others is also considered forgery and is illegal.
Forging a doctor's signature to get prescriptions for controlled substances is often a more serious crime than simple forging for example a minor absence from work.
The specific legal classification whether misdemeanor or felony for forging a doctor's signature can also differ depending on the state and any details of the case.
laws can vary in different states but in most states forging a doctor's signature is always a felony.
If you're charged with a misdemeanor for forgery the sentence can be up to a year in jail, substantial court fees and fines and restitution to the victim.
If you're charged with a felony for the forgery you could be sentenced up to 3 years in prison along with fines and restitution.
Forgery is never a good idea whether it's a doctor's signature or other forgery.