Brief Answer:
Take only dextromethorphan syrup.

Detailed Answer:

Hello,

Robitussin syrup is having combination of dextromethorphan (cough suppressant) and Guaifenesin (expectorant).

I think Guaifenesin is causing coughing in your case. Actually, Guaifenesin is useful when you have chesty cough (congestion).
It is mucolytic and expectorant. So it dilute mucus and makes it easy to cough put.

Since you are not having mucus (congestion), guafenesin is not helpful. Better to take only dextromethorphan containing syrup.
It will supress cough. Do warm water gargles 5-6 times a day. Drink plenty of fluids orally and keep yourself hydrated.

Hope I have solved your query. Wish you good health. Thanks.

Regards,
Dr. Kaushal Bhavsar
Pulmonologist

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