What happens if you stop Phentermine or Zepbound?

Zepbound's label includes a trial that stopped the drug on purpose; Phentermine's does not, which is a difference in what is known rather than in what happens. On Zepbound's label, the people randomly assigned to stop gained 14% of body weight over the following 12 months while those who carried on lost 5.5% more. That figure belongs to Zepbound's trial, so this page does not apply it to Phentermine.

Is Phentermine or Zepbound better for weight loss?

Only Zepbound has a trial figure recorded here: 20.9% average weight loss, from its own label trial. Phentermine has none on this site, so the two cannot be compared on effect and this page does not pick one on it.

Which is cheaper, Phentermine or Zepbound?

Phentermine publishes no all-in monthly price that could be verified, so this page carries no comparison rather than an estimate — and an unpublished price is itself worth weighing.

Can you switch between Phentermine and Zepbound?

They are different molecules, so this is a change of medication rather than of brand, and the one you move to is started on its own dosing schedule.

← All providers and medications

Phentermine (generic) vs Zepbound (tirzepatide)

Verdict

Neither comes out ahead on the axes compared here. Zepbound scores higher on the published halfrx formula, 7.3 against 3.1; the formula takes in more than this table does.

Computed from the stored fields, so it moves when a price does. Nothing here is written by hand or by a model.

Zepbound (tirzepatide) or Phentermine (generic): which one has a price?

Only Zepbound (tirzepatide) has an all-in figure here: $299–$699 a month, read from its own page on 2026-08-19. No price is published here for Phentermine (generic), so price cannot decide between them. Every axis below is computed from stored fields rather than written by hand, so the comparison moves when a price does. Neither carries an FTC action, a state action, a BBB F grade or an FDA warning letter. One is branded and the other generic, and they are different drugs, so the gap is the molecule as well as the price. The published formula scores them 3.1 and 7.3 out of 10, arithmetic over the same stored fields rather than an editor's preference. Cost here is scored on what a patient pays once titrated, never on the entry price, because an introductory rate covers the months before anybody has reached the dose they stay on.

Head to head

Phentermine (generic) against Zepbound (tirzepatide), one row per axis, with the side that wins each axis marked.
What decides it	Phentermine price unverified	Zepbound $299–$699/mo
Entry priceNot comparable — Phentermine (generic) publishes no verified all-in price.	not comparable	not comparable
Regulatory and complaint riskNeither carries a lawsuit, an FDA warning letter or a supply-risk flag.	tied	tied

What the trials, the titration and a year actually say

Axis	Phentermine	Zepbound
The trial that figure comes from	The label does not carry a modern trial figure; it states in its own words that the usefulness of this class 'should be measured against possible risk factors', and it is not recommended for long-term use.	SURMOUNT-1, Study 1 on the label, 72 weeks: 20.9% mean weight loss at 15 mg against 3.1% on placebo, intention-to-treat.
Maintenance dose	37.5 mg once daily, in the morning	5 mg, 10 mg or 15 mg once weekly
Earliest week you reach it	—	week 5
Year one at the published price of every step	No titration prices recorded	$5,637

The year-one figures walk each titration step at its published price and count the thirteen fills a 28-day month bills. The full arithmetic is published.

Go to Phentermine ↗Go to Zepbound ↗

What separates these two, price aside

These are different molecules — phentermine against tirzepatide — so this is a comparison of two treatments, not of two prices for one.

Multiple generic manufacturers

Phentermine (generic)

3.1

Price unverified

Controlled substanceWeaker moleculeNo published priceStrict dosing rules

Eli Lilly

Zepbound (tirzepatide)

7.3

$299–$699/mo all-in

checked 2026-08-19, 50 days ago, past the 45-day window

Price conditional on refill timingBoxed warning

Phentermine (generic)

Pros

+Pharmacies acquire it at $0.068 a tablet, about $2 for a 30-day supply, under the federal NADAC survey. That is what a pharmacy pays, not what it charges you.[2]
+Covered by more Medicaid programs than any GLP-1 — seven states cover an older weight-loss agent while covering no GLP-1 at all.[3]
+One tablet each morning, no injection, no titration schedule beyond a possible half dose.[1]

Cons

−Approved for a few weeks only. Every monthly subscription selling it as ongoing therapy is using it off the label's own duration.[1]
−Schedule IV controlled substance, chemically related to the amphetamines, with the dependence warnings that follow from that.[1]
−Stopping after prolonged high dosing brings extreme fatigue and depression, per the label — which is a hard exit from a subscription that sold it as maintenance.[1]
−No cardiovascular outcome data of the kind the GLP-1s now carry, and contraindicated where blood pressure is uncontrolled.[1]

Sources

Zepbound (tirzepatide)

Pros

+Highest average weight loss of any approved medication — around 20.9% at the 15 mg dose in SURMOUNT-1.[2]
+Dual GIP/GLP-1 agonist, a different mechanism from semaglutide, so it often works for people semaglutide failed.[2]
+$299/mo at the starting dose direct from Lilly against a $1,086 list price, with home delivery of vials or pens at the same price.[1]
+Also approved for obstructive sleep apnea, which can unlock insurance coverage that a weight indication alone would not.[2]
+If you are commercially insured with coverage, Lilly's card can bring it to as little as $25 a month.[1]

Cons

−Weekly injection with the usual GI side effects — nausea, vomiting, diarrhea and constipation are the common discontinuation reasons.[2]
−The advertised $449 ceiling is conditional. It holds only while you refill within 45 days; lapse once and 10 mg and above cost $699 — 56% more than the number in the advert.[1]
−Lilly defines a month as 28 days, so a year is 13 fills, not 12. At $449 that is $5,837 a year against the $5,388 that 'per month' implies — $449 of hidden annual cost.[1]
−$1,086 list price if you have no coverage and do not use LillyDirect self-pay.[1]
−Weight regain after stopping is the documented norm, so the cost is ongoing rather than a course.[2]
−2.5 mg is a starting dose and is not approved for maintenance, so the $299 headline is a price nobody stays on.[1]

Sources

Phentermine side effects →Zepbound side effects →Phentermine dosage →Zepbound dosage →

Comparisons are computed from the fields on each card, not written by hand or by a model, so they change when the data does. Information only, not medical advice. halfrx may earn a commission from some links, which never changes what a comparison says.

Everything on Phentermine

Everything on Zepbound

Phentermine against Zepbound, answered

What happens if you stop Phentermine or Zepbound?: Zepbound's label includes a trial that stopped the drug on purpose; Phentermine's does not, which is a difference in what is known rather than in what happens. On Zepbound's label, the people randomly assigned to stop gained 14% of body weight over the following 12 months while those who carried on lost 5.5% more. That figure belongs to Zepbound's trial, so this page does not apply it to Phentermine.
Is Phentermine or Zepbound better for weight loss?: Only Zepbound has a trial figure recorded here: 20.9% average weight loss, from its own label trial. Phentermine has none on this site, so the two cannot be compared on effect and this page does not pick one on it.
Which is cheaper, Phentermine or Zepbound?: Phentermine publishes no all-in monthly price that could be verified, so this page carries no comparison rather than an estimate — and an unpublished price is itself worth weighing.
Can you switch between Phentermine and Zepbound?: They are different molecules, so this is a change of medication rather than of brand, and the one you move to is started on its own dosing schedule.