I've been taking Mounjaro injections for about 2 months now, so hopefully I can answer those questions.
The two injections available are Wegovy and Mounjaro. Wegovy is semaglutide which is the same drug in Ozempic but in tbe UK at the moment it is branded Wegovy. Mounjaro is a drug called tirzepatide. It's newer than semaglutide and seems to be more effective for weight loss and have fewer side effects. Both drugs have a similar action on the body, mimicking what happens after you eat, to reduce your appetite and make you feel fuller for longer.
Which one you pick is up to you, but I chose Mounjaro because of its apparently greater effectiveness and fewer side effects. There have also not been any supply issues with it so far in the UK.
In terms of reputable sources, there are many online pharmacies that will prescribe it. Big ones like Boots, Asda, Lloyds, and others like Pharmacy2u, MedExpress, Voy. Any pharmacy advertising them should be regulated by the GPhC (General Pharmaceutical Council) and should give their GPhC reg number and details on their website, along with the named responsible pharmacist. They should ask for full medical details, and current photos of you, so they can see your figure and see you standing on scales. Some places offer counselling, online support, a subscription service, referral discounts and so on.
One prescription lasts for a month or so. It's an injection pen that contains 4 doses of the drug, that you self inject once a week on the same day. If you have a holiday or an issue that means you need to change the day you can move it a couple of days forwards or backwards. The instructions that come with the injection pen will explain it all.
Importantly, you need to have a BMI of over 30 to be prescribed these injections. Sometimes they will be prescribed if your BMI is over 27 and you have weight related health conditions but that's quite unusual.
Hope that helps!