Allergic reaction using sensodyne toothpaste. what is a good alternative?

Depends for what : If its tooth sensitivity any toothpaste with 5%potassium nitrate would do the same for sensitive teeth.

Toothpaste allergy: Using a toothpaste without sodium laurel sulphate.

Discuss options: It's difficult to recommend an alternative. Most desensitizing toothpastes have multiple ingredients & most are common to all. You can't be sure which specific ingredient you are allergic to. What was your reaction & how do you know it's the toothpaste? Tom's of maine and mi paste might be alternatives, but discuss with your own dentist as to need and options.

Fluoride gel: You should be tested. If you are not allergic to fl you can use stannous Fluoride gel instead.

Enamelon: Find out from your allergist or dermatologist which components that you are allergic to first. If not allergic, try Enamelon, available through dentists only.

Change Toothpaste: There are many different types of toothpaste available in the market for tooth sensitivity and extra cavity protection too. For example: Pro-enamel, Colgate Prevident, etc.