Does Phentermine Expire or Go Bad? (how long?)

January 9, 2024 |

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This article contains all the information about the efficacy and safety of expired Phentermine.

Yes, like all other pharmacological formulations, Phentermine pills do have an expiration date.

But if you find a whole bottle of diet pills in your closet that expired only a few weeks ago, can you use them? Are they still effective?

Is it safe to use expired appetite suppressants?

What would happen if you took expired Phentermine?

Find answers to all of these questions and much more related to the expiration of appetite suppressants below. Starting with how long your weight loss pills remain effective;

How long is Phentermine good for?

Phentermine pills usually expire after two years of manufacture.

So, suppose you buy a medicine lot that was manufactured about one and a half years ago.

In that case, you can still reap the benefits to their fullest potential because the drug manufacturer’s expiration date is an assurance that the medicine will supply its active ingredients at 100% of their potency until the mentioned day.

What happens when Phentermine expires?

Phentermine can lose its efficacy after the expiration date. After the expiry date, the potency of drugs’ components may reduce, making them less effective.

Another risk associated with expired medicines is the chemical degradation of their components. This can lead to harmful consequences because the degraded products can be toxic.

Although there is no specific data available to support that phentermine pills undergo degradation, we cannot let go of the suspicions about its expired stocks.

Also, the efficacy of preservatives added to pills and liquid drugs is not guaranteed after expiration.

These stabilizing compounds are normally inert (i.e., they do not interact chemically with other components), but they prevent the degradation of drugs’ active ingredients by heat, moisture, chemical agents, and microbial activity.

If the drug preservative loses efficacy, fungal or bacterial microbes may grow upon your medicines. Likewise, these drugs become more vulnerable to heat or moisture damage.

In such cases, your pills may become discolored or powdery.

Nevertheless, you cannot say anything about the efficacy of drugs even if there are no physical signs of degradation unless you get them tested by a pharma laboratory.

Hence, it is better to avoid using expired diet pills and discard them properly.

Does Phentermine become toxic after it expires?

No, there is no evidence suggesting that expired Phentermine becomes toxic.

However, the food and drug regulatory authority (FDA) and healthcare professionals recommend that you avoid using expired medicines because there is no assurance related to their safety and efficacy.

Risks associated with expired drugs

As mentioned earlier, expired drugs are vulnerable to chemical degradation and microbial growth, making them toxic and harmful.

Hence, it is a risk to take expired drugs since it can make you prone to the following side effects.

Drug Resistance

As we discussed in the previous section, the potency of drugs decreases gradually after the expiration date.

The disadvantages of such medicines are two folds. Firstly, they do not treat the disease effectively for which they are used. Secondly, your body gradually becomes resistant to the activity of low-potent drugs.

Hence, your body systems would not respond to the drug after some time, and you would be forced to take the higher dosage.

Drug dependency

Drugs containing stimulatory compounds can slowly make you dependent on them.

That is why the risk of drug abuse increases if you consume, expired appetite suppressants that have semi-potent, neuro-stimulatory compounds like Phentermine.

Due to their lower potency, you would not notice immediate effects. Still, you will find it hard to carry on everyday life activities after some time without taking such stimulants.

Bacterial infections

Expired suspensions and tablets can act as a suitable medium for the growth of bacteria and fungi. However, these microbial colonies may or may not be visible to the naked eye.

Hence, if you take an expired drug, it may contain a bacterial spore that can eventually make you sick with a bacterial infection.

Kidney and liver damage

In some cases, expired medicines can cause severe damage to vital organs.

For example, expired tetracycline was found to impair kidney tubules, resulting in poor renal functions.

Similarly, liver tissues can also be damaged if you ingest expired medicines because most drugs are metabolized in the liver to activate their main ingredients inside the human body.

Does expired Phentermine still work?

Yes, Phentermine would still work after the expiration date has passed, but its efficacy may gradually drop with time.

According to research, most drugs retain a high potency of active agents after expiration. It was found that amphetamines and aspirin are two such compounds whose potency quickly decreases after the expiry date.

As Phentermine is also an α-methylated amphetamine, we can say that its potency would decrease after expiration.

Resultantly, you may not get the best results if you use expired phentermine pills.

So, why take the risk on health safety?

Also read: Why is Phentermine not Working for me? (Explained)

Does Phentermine lose its effectiveness over time?

Phentermine starts losing its efficacy after three to six weeks.

Over time, it becomes less effective for your body because it is a sympathomimetic amine and has stimulatory effects.

As with other stimulants, your body slowly gets adjusted to the regular dose and stops responding to the drug after some time. Resultantly, you feel the urge to increase the quantity.

This scenario leads to drug dependency if you keep taking the dose for a long time. This is one of the reasons why Phentermine and other amphetamine drugs are only prescribed for a short period.


The famous appetite suppressant- Phentermine is an effective but short-term medicine only. This is because it starts losing efficacy after a few weeks due to its stimulatory nature.

Like other formulations, it carries an expiry date, and you should not be tempted to use it after that time as the usage of expired medicines is potentially related to many health risks.

To ensure children, pets, and personal safety, you are advised not to keep a stock of outdated medicines. Instead, discard the expired pills and suspensions according to guidelines provided by FDA and get yourself a new batch.

Related: Clenbuterol vs Phentermine: Which is Better?

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