Since I've been seeing a lot of fearmongering about DIY testosterone again, here's a list of the downsides to getting a prescription instead of DIYing.
Cons that can come with getting a legal prescription for testosterone instead of DIY:
Having to answer invasive questions and be assessed as to whether you are trans enough for a prescription.
Being added to a registry of users of controlled substances.
Prohibitive costs, depending on the medical system/insurance/rules about meeting with a specialist. Vials of T sell online for less than £40/$55 (200mg/ml 10ml vials), with no upfront cost of seeing a specialist or attending appointments.
Less choice over type of testosterone and method. Your doctor may insist on injections over gel, on a long-lasting form of T like Sustanon with higher peaks and lower troughs.
Lack of control over dosage. If your doctor wants to underdose you, they can, and on low-dose T you will have slower changes and less of them.
Being denied your testosterone by a pharmacist on ideological grounds, after it is prescribed.
Getting misgendered at the pharmacy or by wherever you collect your prescription from.
Surveillance by doctors and frequent unnecessary check-ins which waste your time.
No ability to stockpile. Unless you are taking a lower dose than prescribed, you cannot stockpile your T so that you have some to cover shortages or tide you over if your prescription is cancelled.
I'm sure there are plenty more. These are the ones that occur to me off of the top of my head.
There are of course many risks which are similar for both DIY and legal prescriptions - such as being arrested for possession of it! Despite its status as a controlled substance, it is rare for transmasculne people to be arrested for possessing it. In the cases where they are, it's typically because police claim to believe a vial is drugs, and it is uncovered while the person is travelling. To avoid this, only take your T out of your safe place (like your bedroom in your home) if you have no other option.
Look into the options near you, resist the appeal of an authority figure permitting your transition, and make the choice that works best for your HRT journey.