My dog is sick and sad after being left alone. Is it drugs or anxiety?

Hopefully, it is due to the stress of your absence and not a drug toxicity. Monitor him for lethargy, ataxia (wobbly gait), drooling loss of appetite, dilated pupils or rapid heart rate/breathing. These can all be signs of drug ingestion.
Try withholding food for 24 hours. Allow small amounts of water or unflavored electrolyte replacement fluid. Resume feeding a bland diet in small, frequent amounts until the stool is normal before slowly transitioning to his regular diet.
A bland diet can be plain white rice mixed with either cottage cheese, boiled boneless/skinless chicken breasts, cooked/drained ground turkey or cooked/drained ground beef.
Gradually acclimate him to your absence by leaving for ever increasing periods of time. Prepare to leave several times during the day. Leave sometimes, sometimes just put your stuff away and stay. Do not make a big deal about leaving or arriving. Ignore your dog for 15 minutes or until he calms down upon your return. Use special toys or treats only when you leave. Take them up when you get home. Pheromone products such as the DAP Diffuser or Adaptil collar may decrease his anxiety while you are gone.