They are both antihistamines but the difference is in parenthesis below.

Dexchlorpheniramine - Clinical Pharmacology

Dexchlorpheniramine maleate is an antihistamine beside anticholinergic (drying) and sedative side effects. Antihistamines appear to compete with histamine for cell receptor sites on effector cells.

{This compound have an anticholinergic and sedative side effect}

Chlorpheniramine maleate,

Deconamine is an antihistamine and decongestant used for the temporary relief of harsh runny nose, sneezing, and nasal congestion caused by upper respiratory infections (the common cold), sinus inflammation, or hay frenzy. It is also used to help clear nasal passages and shrink swollen

{This one has a decongestant but no anticholinergic or anaesthetic effect}

Source(s): Nursing drug resource

They both are antihistamines in cold remedies like Chor-Trimeton.

The "dex" is an abreviation for "dextro" or right. This is a chemical prefix meaning this is a different edition of the standard chlorpheniramine with one of the methyl complexes turned in an unusual way (to the right).