If it really worked, it would become a (prescription) medication and you'd be able to buy it at 30% of cost.
この日本語の文章で「薬」は「処方薬」のことですね。そうじゃないと3割負担で買えないですね。
文法的に、この文章は「If.... then ...」という形で、「then」のあとで「would」を使いますね。「It's」ではなく「it would become」になって、「you can」ではなく「you would be able to」になります。複雑な文法ですが、英語は日本語より時制の種類が多いです。これは「仮定法過去」(conditional type two; contrary-to-fact conditional) と言います。
If those supplements really worked, I think they would become more like a prescription drug and you could buy them using your health insurance, then you would only have to pay 30%.
この場合、次のような言い方ができますよ。
ーIf those supplements really worked, I think they would become more like a prescription drug and you could buy them using your health insurance, then you would only have to pay 30%.
「そのサプリが本当に効くなら、処方薬になって保険が効いて３割負担になってるだろう」
to work「薬などが効く」
prescription drug「処方薬」
ご参考まで！
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