Thanks for your post. PrEP is growing popularity and we're getting a lot more questions about how it should be taken.
It sounds like you've bought from an online pharmacist, which means you haven't had time to discuss your PrEP regime with a doctor or nurse. Thankfully there are a lot of people out there in your situation who are buying PrEP online. Terrence Higgins Trust, a longstanding HIV and AIDS charity has the exact advice you're looking for on its website for people in your situation :
"In clinical trials PrEP has been used in two different ways:
taken regularly (one tablet per day). only taken when needed (two tablets two to 24 hours before sex, one tablet 24 hours after sex and a further tablet 48 hours after sex). This second method is often called ‘on-demand’ or ‘event based’ dosing.
Both methods have been shown to be very effective, although on-demand dosing has only been studied in gay and bisexual men."
The page also mentions that even if you're taking as prescribed, you should still go to your local STI clinic so they can perform tests on you to be sure the medication isn't hurting you. The antiretrovirals can have serious side effects and affect kidney function, so be sure to go in and get monitored!