Will you always have tonsillitis on only one side?

Rarely: Inflammation of the tonsils or tonsillitis is almost universally on both sides. There are cases where one side may have a more complicated problem such as an abcess. See your doctor for help.

Not usually: Tonsillitis often affects both tonsils. If only one tonsil is big or hurting that may be more concerning and your should get checked out. There could be an abscess in the peritonsillar space, especially if it's hard to open the mouth. There is also an extremely rare situation of tonsil cancer or lymphoma of the tonsil that also usually is only present on one side.