I would explain the entire situation to the Background Investigator in detail.  At the time of usage you were under the direct supervision of a pharmacist, which in your country of residence was completely legal and normal.  I would also stress that you have never taken steroids in this country, and that those days of augmenting your weightlifting with drugs (legal or not) are certainly over.  

If you lie about this, they will find out during your background investigation, guaranteed.

Good luck.

Kona