粉薬って英語でなんて言うの？

You can simply say, "I took a powdered medicine for a headache." It is not a tablet, pill, or liquid, you can mix it with water.

Medicine granules' or 'oral powder' are two names for the same thing. A pharmaceutical drug that comes in powder form usually contained within a glass bottle or a neutral plastic tub.
Some medicines are available in powder forms so that the doctor or patient can measure a precise dosage.

Medicine that is in powder form is known as simply 'a powder' (if you can see the medicine packet or container). Alternatively, it may just be called 'medicine' - exactly like a liquid. If the context is unclear, then it may be necessary to state 'powdered remedy.'
'John took a powdered drink for his cold.'

Medicinal powder is usually added to hot or cold water to make a drink from the powder. It is stirred well and digested orally, commonly used to treat a cold, flu or sore throat. Hot powdered medicine drinks are thought to help ease symptoms of a sore throat.

"Powdered medicine"
Powdered is the term given to something that is not liquid or solid, powdered can also be referred to as granules, often very very small granules of a particular food/medicine etc.
"Medicinal powder"
medicinal is the term given to something that is a medicine and has healing properties such as medicinal marijuana.

This type of medicine is in the form of powder and can come in a variety of color shades.
It is easy to consume for people like children, the elderly or the drastically sick.
It can easily be dissolved into water or any drink as well as added into food for administration.

Administer: to give medicine
Shades: gradients, different colors

"I "just had some powdered medicine for this headache, Ï've been in pain all day!"