What is Doctor’s Best Calm with Zembrin? Doctor’s Best Calm with Zembrin helps combat stress, tension, and occasional anxiety. It helps increase attention, focus, motivation, and concentration. Zembrin is a patented standardized extract of Sceletium tortuosum; its use has been documented for more than 300 years. It has been clinically shown to enhance cognitive function and is cultivated in an environmentally stable process.