Loaded pens vs vials

Vial — you mix it

A glass vial of freeze-dried powder with a rubber stopper. You add bacteriostatic water yourself, which means you choose the concentration.

You decide the strength. Same 10 mg vial, 1 mL of water gives 10 mg/mL; 2 mL gives 5 mg/mL. Half the concentration, twice the clicks for the same dose.

Finer control if you want it. More water means more clicks per dose, so each click moves the dose less. That matters at small doses.

Longer shelf life before mixing. Powder is stable; once reconstituted the clock starts.

More to get wrong. The maths, the mixing technique, and drawing it into a syringe or pen cartridge.

Use the calculator normally: vial mg, water added, dose wanted.

Loaded pen — already mixed

A pen supplied with the cartridge filled. Nothing to reconstitute, nothing to draw up. Dial the clicks and inject.

The concentration is fixed. Whoever filled it chose the water volume. You can't change it without emptying the pen.

Nothing to mix. No bacteriostatic water, no syringe transfer, no technique to get wrong.

The label is doing a lot of work. A pen sold as "20 mg" tells you the total content, not what a click delivers. Those are different numbers.

You still need the click maths. Just from the other direction — not "how much water", but "what does one click give me at this fill".

Enter the pen's total mg, then the fill volume in the water field. If the fill volume isn't stated, ask the supplier — without it the click figure is a guess.

Total content (mg)

How much compound is in the whole pen. This is the headline number on almost every pen — "Retatrutide 20 mg" means 20 mg across the entire cartridge, not per dose.

Fill volume (mL)

How much liquid the compound is dissolved in. Often left off the label entirely, which is the problem — without it you cannot work out what a click delivers. A 3 mL cartridge is not necessarily filled to 3 mL.

Concentration (mg/mL)

Total content divided by fill volume. This is the number the click maths actually runs on. If a supplier states concentration directly, you don't need the other two.

Worked example

A 20 mg pen filled to 2 mL: Concentration is 10 mg/mL. One click is 0.01 mL, so one click delivers 100 mcg. A 2 mg dose is 20 clicks.
The same 20 mg pen filled to 1 mL: Concentration is 20 mg/mL. One click now delivers 200 mcg — double. That same 2 mg dose is 10 clicks, not 20.
Why this matters: Identical labels, identical compound, and the click count differs by a factor of two. Guessing the fill volume is how people take double or half what they intended.

A pen suits you if

You'd rather not mix anything
You're travelling and want one object, not four
The dose you want lands on a whole click at that fill
You've had trouble with reconstitution technique before

A vial suits you if

You want to choose the concentration
Your dose needs finer resolution than the pen's fill allows
You're dosing small amounts where one click is a big jump
You want the powder to keep longer before you start

Either way

Check the batch COA on the lab's own site
Use assayed content, not the label figure, if they differ
Refrigerate once mixed or once opened
Know your concentration before the first dose