Serious side effects with Allegra and Allegra-D are rare, but milder effects do sometimes occur. Effects you may experience with Allegra include:

• headache
• cold
• back pain
• coughing

If you take Allegra-D, you may also have:

• nausea
• insomnia

Consult with a doctor if any of these side effects become cause for concern or do not desist after a couple of days.