
jemandem zusetzen
to affect
To put a strong strain on someone, cause them difficulties, or affect them strongly emotionally or physically.
Die ständige Kritik hat ihm sichtlich zugesetzt.
The constant criticism visibly affected him.
to affect / add
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to affect
To put a strong strain on someone, cause them difficulties, or affect them strongly emotionally or physically.
Die ständige Kritik hat ihm sichtlich zugesetzt.
The constant criticism visibly affected him.

to add
To add or mix something into a mixture, liquid, or substance.
Dem Teig wird noch eine Prise Salz zugesetzt.
A pinch of salt is still added to the dough.

to lose money
To lose money in a business deal or venture; to suffer a financial loss.
Bei dem riskanten Investment hat er eine Menge Geld zugesetzt.
He lost a lot of money on the risky investment.
'Zusetzen' (to affect/add) is a separable verb. In a main clause, the prefix 'zu' comes at the end of the sentence. In subordinate clauses or in the infinitive with 'zu', the verb stays together.
Der Lärm der Baustelle setzt den Anwohnern sehr zu.
The noise from the construction site is really affecting the residents.
You can recognize the meaning from the grammar. 'Jemandem zusetzen' (to affect someone, with dative) means to put strain on someone. 'Etwas zusetzen' (to add something, with accusative) means to add something. 'Geld zusetzen' (to lose money, with accusative) means to make a loss.
Die Hitze setzt mir heute wirklich zu.
The heat is really affecting me today.
When 'zusetzen' means 'belasten' (to burden/put strain on), the person who is affected is always in the dative. The thing causing the strain (e.g. stress, noise, illness) is the subject of the sentence.
Der Tod seines Hundes hat ihm schwer zugesetzt.
The death of his dog hit him hard.
In the meaning 'hinzufügen' (to add), the verb is often used in the context of cooking, baking, or chemistry. Typical things that are added are 'Salz' (salt), 'Zucker' (sugar), 'Gewürze' (spices), or 'Zusatzstoffe' (additives).
Dem Trinkwasser werden hier keine Chemikalien zugesetzt.
No chemicals are added to the drinking water here.
to burden / charge
Both verbs can say that worries or problems have a strong negative effect on a person. “Belasten” describes this burden more generally, while “jemandem zusetzen” emphasizes the strongly felt, often personal effect and requires the fixed dative form. “Zusetzen” is therefore especially suitable when someone is clearly suffering as a result.

Die Sorgen setzen ihm seit Wochen zu.
The worries have been taking a serious toll on him for weeks.
Die Sorgen belasten ihn seit Wochen.
The worries have been weighing on him for weeks.